How can I have one circuit always utility powered while all other circuits are either generator or utility powered?

While I'd normally recommend having your standby loads on a subpanel...

My normal recommendation to folks who want a backup generator, whether portable or permanent, is to move their standby loads into a subpanel. This is how the "big boys" do it, as it provides a built-in level of protection against generator overload from large/unnecessary loads (such as an electric range in a house, or a large motor load in a hospital), and also can be used with equipment suitable for a bonded neutral generator, which is important as portable generator sets generally come from the factory with their neutrals bonded so that they can meet OSHA specs for jobsite use.

If you were to take this up and have the standby loads on a subpanel, then this problem would be a nonissue as you could simply have an extra branch circuit run to the subpanel for the purpose of having a "utility on" pilot light fitted there.

However, there is a way out of your situation while maintaining the use of a CSED

The good news is that thanks to the use of line-side taps on solar systems, there are a couple of possible solutions to your puzzle of how to get that single utility circuit in a whole-house interlock setup. While your proposal #2 with tap lugs on a 200A meter base can be made to work using a Milbank lever bypass socket and a set of Milbank K5022-INT tap lugs, the coming 2020 NEC exterior disconnect requirements make a CSED (meter main) a simpler choice for this application. Furthermore, thanks to the aforementioned line-side solar tap stuff, Square-D makes a lever bypass meter main that has support for a line-side tap lug kit, namely the RC816F200SLS. While something you'll have to special-order from a supply house, you can fit it with a SR69064A lug kit that lets you make a line-side tap off the busses to the main breaker without voiding the UL listing.

With this out of the way, we can use a 1" nipple from the bottom right of the meter-main to the bottom-left of a 30A fused A/C disconnect, with a pair of 10AWG copper THHN wires for hots inside the nipple, and bonding locknuts with 8AWG copper ground jumpers on both ends as this is service entrance wiring. In this disconnect, we can fit a pair of 15A or less cartridge fuses to provide overcurrent protection for the alarm circuit; from there, you can make your indicator as sophisticated as you wish.

As to that alarm...

While there are a variety of solutions to the alarm problem, the simplest possible alarm one can implement that won't whine at you for the entire time the power is on consists of an NEMA 4X enclosed SPDT relay with a 240VAC coil, an IP65 rated 240VAC illuminated SPST 22mm pushbutton, and an optional IP65 rated 240VAC audible alarm device (buzzer) that fits a 22mm (1/2" trade size) KO. In particular, a Functional Devices RIBH1C-N4 can be used for the relay, an IDEC HW1L-M1F22QD-G-240V (or -R, or -Y, whatever color floats your boat) works for the illuminated button, and an Auer Signal M22-30K-240VB will serve as the buzzer in this application. These parts are all UL listed as industrial control devices, so there are no Code issues with using them in this application, and 22mm devices fit into 1/2" trade size (7/8" actual size) knockouts, which your average A/C disconnect sports plenty of.

Hooking this up goes as follows, using 14AWG THHN and ordinary wirenuts:

• First, the various devices get mounted. The relay's nipple goes in the leftmost bottom knockout on the disconnect, while the alarm goes in the rightmost bottom knockout if used, and the right-side knockout gets used for the button.
• Then, we start wiring things up with the pigtails from the load side of the disconnect. One of these pigtails is wirenutted to the yellow common wire on the RIB and a pigtail to one of the NO terminals on the switch, while the other LOAD pigtail from the disconnect is nutted to the white/yellow coil wire on the RIB, a pigtail to one of the light terminals on the switch, and a pigtail to one of the terminals on the buzzer if the audible alarm is used.
• We then move onto the other coil wires on the RIB: the white/blue coil wire gets capped off as it's not used here, while the white/brown coil wire gets nutted to a pigtail from the other NO terminal on the switch and the orange NO wire on the RIB.
• Finally, the blue NC wire on the RIB gets nutted to pigtails from the remaining light terminal on the switch and the remaining terminal on the buzzer, if present. Any remote alarm indicators can be connected in parallel with the light and audible alarm, as well. Just keep in mind that this remote indication would be a 240VAC, Class 1 control circuit, so you'll have to run it using 14/2 NM with the ground wire in the NM cable connected to the grounding block on the AC disconnect box.

The basic theory behind this is that we are using the relay as an inverse stick relay, allowing it to store the "alarm reset" state. Starting with the power off, the alarm signal is connected to incoming power via the NC (normally closed) contact on the relay, allowing the alarm to turn on when power returns. In order to reset the alarm, pushing the button applies power to the relay coil, causing the relay to energize its NO (normally open) contact instead, which provides power to the relay coil after the button is released, and thus "sticks" it in the "alarm reset" state until power is removed from the circuit again.