This shot was taken with a 105mm f2.4 67 lens on a 645:
http://flic.kr/p/rhtKkv
Here's the 75 f4.5 with extension tubes:
http://flic.kr/p/qGabe9
Afaik, she only has samples from 67 with extension tubes. If you can find an adapter cheap enough (that's the tough part!), then yes it's a workable solution. You might want to try with a helicoid tube (although the normal set are probably cheaper).
One advantage of the 67 primes is that they tend to be cheaper than 645 ones. One disadvantage is that they are huge!
On the 67 front, you might want to look at the 100 or 135 macro lenses (the 100 is better), or alternatively hunt down the 120mm 645 macro (in either A or FA flavours)