Can adjectives be used like this? (genitive)

Yes, in general, those are acceptable. Whether they are a good choice from the point of view of clarity and style will depend on the context and the writing skills of the author. They are not particularly rare, but they are also not particularly common, in the sense that it is quite likely that you may go through several newspaper articles without encountering any.

Discussion

According to CGEL, when the dead, the long lost, etc. are used as a noun phrase (NP), that type of NP is called a fused modifier-head. There are various types of fused modifier-head NPs. Here we are talking about a type that we might call the 'adjectival type', i.e. the type that consists of a definite article and an adjective, plus possibly a modifier in between. The only general constraint on the type of adjective allowed is that it must be possible to use the adjective attributively (CGEL, p. 529).

CGEL doesn't explicitly comment on the genitive form of these adjectival fused modifier-head NPs. But it is not uncommon to see them in the genitive.

Here are some examples of usage. First, from the Corpus of Contemporary American English (COCA):

Police said two more bodies were found in one of the dead's apartments.
…which limits the poor's access to the market…
…can't be brought up if their only purpose is to provide evidence of the accused's character.
…paid time off for mothers and fathers at any time during the newborn's first six months.
…the mother wrapped her forearm around the eldest's neck.

And from the British National Corpus (BNC):

It was found in a cardboard folder in the back of the accused's car.
It is not because of the poor's isolation from the modern sector that they remain poor.
…with the provincial's enthusiasm for champagne…
…flowing from the insured's death.
He was opposed, to shinty and the young's enjoyment in general.

The phrase the long lost's doesn't occur in these corpora, but it does occur e.g. here:

Occurrences beyond human control such as a chance meeting, a hurricane, the long lost's appropriate return, if they come early in the story and are apparently of no great consequence to the plot, have no effect on plausibility.
^{From Kenneth Payson Kempton, The Short Story (1954) (link)}