Is this anything to do with Cassivellaunus? In Welsh this would be Caswallawn or Caswallon and it appears the form in Modern Breton would then be Kawalon or Kawalen.
That would be more likely. I was confusing the Cornish sound change /t/ > /s/ with Welsh and Breton /t/ > /d/, cf. Welsh & Breton tad, Cornish tas (so Cornish has Kaswallon for both Cadwallon and Caswallon of Welsh).
Still can't help with the pronunciation though, sorry. There doesn't seem to be any info on a Breton version on the Breton Wikipedia.
Greetings! Kadwallen (also caduuallon, and Catwollen) is the Breton name for 7th C Welsh king (and saint) Cadwallon. The name means something along the lines of "Valorous in Battle." Cadwallon ap Cadfan (full name) was king of Gwynedd, Wales until his death in battle in 634 AD. As you might imagine, with a name like "Valorous in Battle," more than one person had it, and spindlemoss pointed out the similarity as well to "Caswallon". This list of British monarchs shows the popularity of both (and other) variations: http://www.angelfire.com/ego/et_deo/britishmonarchs.htm
With all the variation of this name, it's possible that a translation other than "Valorous in Battle" may apply at some point, too.
With all the variation of this name, it's possible that a translation other than "Valorous in Battle" may apply at some point, too.