I'd like to see the possibilities. But the truth is I can buy a used
laptop for less money than a mini-itx motherboard with memory, so
there's no way the economics favor the homebuilt.

The thing is with crictical systems we want redundancy (backup), and
this includes PC's.
Besides navigation, PC's can be used for a number of 'black box'
applications, including AIS and fish finder to name a couple. Also,
there is the SSB radio interface with a PC. So a back-up system is
even more vital.

So you really need to double the cost differential between a laptop
and other options when you're calculating the economics of it.
Roughly, you can buy two laptops for the cost of a single mini-itx
system. The laptops will be slightly faster and use slighlty more
power. Many laptops seem to survive ok in the marine environment
without being sealed even if it was possible to do. Of course another
option is to have a mini-itx with a laptop backup(-:.
Cheers, Jim