Using Comfrey for Grass Exclusion

Our community garden raises organic produce for our local food bank in raised beds. We have an ongoing problem with invasive Bermuda grass from outside the garden area. I attended a class recently in which the grower had planted comfrey all around the borders of his garden because he said that it suppressed grass from infiltrating (he was not dealing with Bermuda, however).
I suggested this idea to our garden director, but am wondering (1) if it is likely to be effective for this purpose, and (2) whether any particular cultivar would be better than another? I have heard from others in this area that comfrey grows into a huge 5ft x 5ft plant which would be too large for our purposes. We do make our own compost, so it would be useful as well for that. Do you have a recommendations? I had been considering the Bocking 14, as I had always heard that it was the best for compost, but am hesitant to plant something that is going to be a bigger problem than the Bermuda!

Comfrey can itself be invasive; so you may be replacing one evil with another, depending on how you look at the situation. Generally, those who are not that devoted to making medicines from their garden, and certainly those not looking for a cheap source of biomass for animals or compost, are likely to look upon comfrey as a nuisance as it springs up in different places in the garden. Once established, comfrey is hard to get rid of; so a decision to grow it must be made carefully.
All of the major varieties of comfrey (common, Bocking 4, Bocking 14) have the same effect: they form dense patches that crowd out virtually everything else. For the purposes you have in mind, any one of these varieties will work, but Bocking 4 probably would produce the most biomass for compost.