>> og eru slíkt heimskir menn sem þú ert

> But in this case "heimskir menn" is in the nominative, not the

genitive, so I don't think there is a valid comparison between these
constructions.

You're right, it's not exactly the same construction as `margt manna'.
On the other hand, I still think `slíkt' here would be better
characterised as an adjective or pronoun than as an adverb. One
reason is that examples of this sort with `slíkt' can very often by
paralleled with examples where a pronoun, `þat' or `þetta', is used
instead.

(1) a) eru slíkt firn mikil; b) þetta eru firn mikil
(2) a) og eru slíkt gersimar miklar; b) þetta eru inar mestu gersimar
(3) a) slíkt eru mikil undr; b) þetta eru mikil undr
(4) a) slíkt eru in mestu níðingsverk; b) þat er níðingsverk
(5) a) slikt eru yrkisefni; það er gott yrkisefni (Modern Icelandic
equivalent offered by CV).

I don't know of examples like this where `slíkt' could be replaced by
an indisputable adverb. In these examples, the singular pronoun is
the subject of the copula, even though `eru' is plural (except in 4.b
and 5.b), taking its number from the complement. This is typical in
such constructions where there's a disparity in number between subject
and predicate nominal.

(6) þat eru æðar tvær `those are two eiderducks'
(7) þat eru hrímþursar `those are frost giants'
(8) þat eru góðar árar `those are good oars'

(9) þat heita Gleipnisvellir `it's/they're called Gleipnir's Plains'
(10) þat heita Kolluvísur `they're called Cow's Verses'
(11) þat heita hof, er goðin eiga `they [those places] are called
temples which the gods own'