Doing a simple search here turns up some others (ignoring the ones with familiar English versions, like Deck the Halls):

Hosanna Fawr (Bethlehem):
http://www.mudcat.org/thread.cfm?threadid=125538
http://www.youtube.com/watch?v=1UyU8Z2FK_s

Wel, dyma'r borau gorau i gyd: New Oxford Book of Carols, #166 (p. 477)


The Welsh for the Poverty carol ("All poor men and humble") is "O deued pob Cristion" (Oh come all Christians). English words (still copyrighted) can be found in this thread:
http://www.mudcat.org/thread.cfm?threadid=107441
Welsh lyrics and another singable translation may be found here:
http://www.angelfire.com/in/gillionhome/Worship/Emynau/ODeued.html


On YouTube you can find some more, like:

Tua Bethlehem Dref (Come from ev'ry land):
http://www.youtube.com/watch?v=JsjCwTOuPhM

Hwiangerdd Mair (Mary's Lullaby) http://www.youtube.com/watch?v=JMH_NAu9czk

Draw Tua Bethlehem (Far Over Bethlehem):
http://www.youtube.com/watch?v=JP6WJ_JfQWo