The Ancient Ilythiiri Plate which I newly acquired has racial restrictions of Elf/Half-Elf only becuase of it's size, but as a certain good player pointed out you can still UMD it.
I'd suggest adding a note of 'Don't use this item with UMD unless you are of the required race' as on the suits of armor made for halflings and gnomes in the underdark.
That lines up with the small stature stuff we had in Sanctuary. Makes sense to me.
I am fine with people UMD'ing this armor as long as they emote it nearly bursting off their fat human frame and looking like a damned fool in it.
However...
It is getting yanked from the treasure drops anyway so this shouldn't be a large issue. Go nuts with it.
...or presumably sagging off their perfectly formed gnomish frame?
My only bard I've ever played UMD'd a piece of gnomish chain for his entire existence! Looked like a darned fool too...