Is it possible in Lightroom to change a develop property of multiple photos by a certain amount?

Ok, I see your problem. NO you can't do that with Sync and get different values. Sync looks at one thing as a whole when you use it. I would do this. Take ONE photo and do the correction you want on it, THEN make that a PRESET to use in development. Do the same for the next photo that has a different value you want, THEN make that a new preset to use in development. Once you have created and NAMED these fine tuned presets, just highlight the photos that need said amount of change, click on the preset you created and named with that needed value and THEN hit SYNC. All the photos that you want to have THAT value only will have it. Then go back and do it all over with the photos that need the next value change and use your next preset you made and named. It seems like work at first, but remember, you created a preset that from now on, you need only apply the preset you have created to use it over and over again... you did all the work only once. That is how I would do it. I hope this helps you. ALWAYS make a preset with your fine tuning work and name it... No need to keep working harder than you have to. Use Lightroom to your advantage, It is made for making ease out of this kind of stuff.

Here is a crude workaround; export all photos you want to adjust to tiff. Import back into library, make your clarity adjustment, select all and sync.

I had the same problem - I wanted to change a certain parameter in several photos to the same value. I found 2 ways:

1. As rob j crowe said - you need to edit the photo not in the Develop tab, but in the Library. There, all change is RELATIVE, not ABSOLUTE.
2. Use the plugin Relative Adjustments.lrplugin. For a start, you can use its trial version (it does the desired effect only up to 5 photos). Here is a detailed demo of how the plugin works.