Sad to say your basses options are quite a bit more limited than if you were playing it in G than they are in D. But are you sure you are? I only ask because you say you're playing a lot of G and D chords; if you were playing in D I'd expect it to be D and A chords. And the modulation is to the dominant, the tonic is where you start from, but saying this will probably just bring down the wrath of those who think ignorance of music theory is a prerequisite for melodeon playing, mutter grumble etc.
Anyway, assuming you are in D there are a few things you can do with that measure. Taking the line "Don we now our gay apparel" you could use E minor for 'Don we now our' or you could use bare fifths with A & E (both bass notes. no chords) for 'Don we now our' and D & A for 'gay apparel' but you'd have to play all this on the draw which requires cross-rowing on the melody side, which is easy enough but you have to watch you don't take the bellows too far. For a faux-early feel parp the bass fifth on 'Don', 'now' and 'our', and likewise in the following bar. The modulation in the subsequent 'fa la la, la la la, la la laa' is tricky, you need to give the impression of an Emajor chord. The best you can probably do is play an E bass with no chord on the third and fourth las. The real problem is the fifth one: the melody note is D so it's got to be press, which means you can't play the E bass but reverting to a D chord sounds terribly weak. One compromise would be to play a B bass and on the D melody row play D and G#, which is the button nearest your chin if your fingers will stretch. Another would be to make virtue of not having the chords and just play a staccato chord/bass/both on 'Don', 'gay', 'fa' with melody only for the rest, and then resume left hand for 'la la laa', which should go A E A so more cross-rowing needed and again the E will have to do without its chord (unless you've got a stop to remove the thirds in your chords or are willing to disarm the third for this occasion, neither of which I like but tastes vary). I hope this helps, good luck.