OTOH, I don't think it's practical for a layman to "reinforce" an
already-built boat by adding layers of Kevlar. It is harder to work with
than glass, and has to be protected from abrasion, so it cannot be used
effectively for the outer layer. Also, it seems impractical to me to add
more layers to an already-built hull...

And I'd agree, as the properties of Kevlar really don't match that well
with glass. It is also a pain to work with and as above, needs to be
protected from abrasion.

Kevlar is nasty to work with, ending up with all those 'fuzzy' bits
when you try and sand the stuff. It can be used where lots of flexing
is desirable but your hull is not really one of those - the right place
is your deck where you trample all over whilst changing sails &
drinking.

If you want a heavier layup, add more glass, otherwise go for a new
hull with a 100% carbon layup in the same weight as glass - it'll be
tough and stiff as old boots if built correctly.

Artie