I’m wondering if this is uncommon, I don’t notice other melodeon or Irish chromatic players on Youtube using the thumb. I play the C#/D and find it very natural and comfortable not only on the bottom row but also on the upper row on the low notes. I got used to using the thumb after spending about three months on the C griff chromatic accordion. (I hit a brick wall with the chromatic but thats another story.)
How about it, anyone else using the thumb ? Could this hamper my playing down the road.
I’ve seen or heard about people using the thumb (to play notes) on diatonics as a matter of normal playing style in other traditions - on 3- or 4-row machines - but never yet in Irish music. I have seen Jackie Daly playing the first part of Scotland the Brave in B major, all on the draw, with his thumb sounding the drone on the low B - as a gimmick, for amusement. And I use the thumb occasionally myself to get the same low B in a right-hand chord at the end of a tune. This is still at the gimmick stage too.
If you used it in normal playing, I have no idea whether it would hamper you. For myself, I can’t imagine getting the kind of woomph I want in my playing without the thumb pressing on the edge of the keyboard. It stabilizes the instrument of course, but can also be used (as a great Québec one-row player, Sabin Jacques, was explaining to me recently) to initiate the attack on accented notes on the press.
Players like Mairtin O Connor and Dave Munnelly get masses of woomph with the thumb behind the keyboard… but in front? You’d need to be tightly strapped in with two straps, I think. Let us know how you get on!
Paddy O’Brien, too. He’s got a wooden bar affixed behind/below the keyboard to keep the thumb from sliding too far under. I got a close look at a workshop with him a few years ago.
When I think woomph I don’t normally think of “B and C” players 
but talking of Paddy O’Brien’s thumb, have a look at this clip - mystifying, besides looking very uncomfortable. I can only assume he’d been having trouble with it behind the keyboard.
Well he has more woomph than me! Yes, the bent thumb looks very odd. And if you look closely at the side view in the video at around 1:32 to 1:37, you can just see the wooden bar I was talking about, visible just above the neck of the guitar.
The wooden bar thing is odd. It’s as though he’s using the same technique as most players - pressing the right thumb, but against the bar, instead of the edge of the keyboard, whereas the other players who put their thumb behind the keyboard obviously manage well without such an aid. It makes me wonder how he ended up playing that way.
Regarding the video clip, if the tip of his thumb were sore for whatever reason, playing with the edge of the thumb like that might keep his fingers in the same position over the keyboard. But I just tried it and I think it would get pretty painful very quickly, unless one were playing in a totally woomph-free manner. I’d find it easier to butt the ball of my thumb against the edge of the keyboard. 
Weird at all events.
Here’s an example of someone using the right thumb on a diatonic: http://uk.youtube.com/watch?v=Bfv6rf5fiQk&feature=related
Yeah, that’s my style exactly, and I’m almost as good as he is 
Seems that it is quite frequent for Austro-Bohemian ( ) style of playing the diatonics:
http://uk.youtube.com/watch?v=PmrOwinz-RI&feature=related
http://uk.youtube.com/watch?v=uQfkATCpW3c&feature=related
http://uk.youtube.com/watch?v=XdTsZQ4_aDU%20 (3:48 and 4:39)
http://uk.youtube.com/watch?v=H65JLABDvdg&feature=related
They’re often even 4 1/2-row boxes, though…