> I know almost nothing about MESS although I've certainly used MAME a lot in the past. I was told to check the list for MESS "undumped" and "wanted" machines, and indeed the one I was wanting to emulate - the Amstrad PC5286 - is on the list
Note that there is no MESS anymore, it is now integrated into MAME. Has been for a few years. So if the list you searched still mentions MESS, it means that it is at least ~5 years old and might be severely out of date. In your case it's correct, but don't rely on it too much for other systems
The list(s) he's mentioned is the "Undumped" ones at "Duke's wiki" of mess.redump.net - which used to be at www.mess.org
before Sean repossessed the domain name (after the "death"/afterlife of MESS.)
I even still actually sometimes update those lists, I think the last time was mid-late 2019. gregf of MAMEWorld had been sending me updates of what to remove, he's good at projects like that.
But generally speaking, all that content at mess.redump.net is better to be updated and merged into mamedev.org / wiki.mamedev.org / docs.mamedev.org and many other places (the "Undumped" stuff -had been- in the progress of being moved to the Undumped Wiki in late 2019), a big project that simply needs a lot more volunteers. People like myself and Firehawke have done what we can but sometimes you simply have to backburner something like that and move onto other things.
Also, happy to see that you've found one of my other haunts, Banjo - this is a better place to discuss these things than vogons.org