Adrenaline currently has a field bb only policy as well. They provide bastards and biovals in differing weights and amounts.
I understand the desire to use bio bb's outdoors, the reclamation fees for removing 'unwanted' plastic from land is incredibly expensive. The insurance policy carried by paintball fields demands that bio-degradable product must be used, I can imagine that the same will hold true for insurance for an airsoft field.
Continuing with paintball as an analogy, almost every paintball field has a 'byop' (bring your own paint) policy; you can bring your own paint, BUT is has to be a certain fill colour and the field has paint available to purchase if needed. This is pretty much exactly what you seem to be setting up; bring your own, buy it from you, doesn't matter as long as the bb's are bio-bastards.
A suggestion for bb's purchased off-site; have them arrive at the field with the bag still sealed. That ensures that the bb's are in fact bastards and of the weight described on the packaging.
I know that some will complain; they want to pre-load their bb's, the have a massive personal stock of non-bio bb's... at the end of the day, you have to do what makes sense for your business.
I fully endorse the idea and look forward to getting to Toronto to shoot.
The Tier One Tactical Operator
Tippmann Arms M4A1
KWA USP .45, KJW P226, KJW hi-cappa