Well that seams to be a nice solution to compile it to procmail rules.
But then why can't I use regexp or copy my incoming emails to different folders.
I can definitely see why DH disallows custom procmail processing, since it is possible to rather stupid things with it. But the current mail filtering solution is frankly not good enough.
As I said before Mail sieve is a standardized way of doing just this sort of thing. Which means that there will be support for it in more than one place (webmail clients, thunderbird, etc) and more people are working on improving it than just another custom mail filter solution. In my experience you never gain anything from reinventing the wheel. Even though we coders sometimes prides ourselves when we have done just that. I guess if you guys at DH get it set up properly you will save time supporting and maintaining your current, as I guess in house ad hoc solution.
As for what sieve version I recommend. I don't know what versions there are, but if you are interested I can ask the sysadmins at my work what they use.
Yet again, thanks for an otherwise outstanding web hosting, if you get this set up properly DH will definitely be the best!