Really? Where are you paying those rates? Dry stone walls needs quite a bit of maintenance if they are to last.
They won't have an SBI number as they would not qualify as an active farmer so can't claim SPS (now BPS). They will be able to claim some of the Pillar 2 funds though.
However, the last thing we need is a towpath forest filling up the canal with leaves. It's bad enough as it is now. If you want a forest, then go to a forest!
ETA - also I think you are confusing "greening", which is an element of the Pillar 1 BPS, with the other funds available under Pillar 2 (which is the 12% you refer to).