A quick google suggests that Hyrdroxyl groups and possibly any water in the bamboo (some part of the sap I guess) react during the chemical process the PU goes through to act as a foaming agent or modify the chemical reaction... I think that might be the problem...
Today I've poured some PU into the perspex channel. I used a wax and hair spray as release agent and did not leave it for long before PU. Now thinking that a hair spray has got water in it:((( That might foam.
Also poured PU into the sealed "rails" on a bamboo leftover. Will report back the results.
Great experimenting and well done on cracking the code.
Did dan mention why they were using PU in production boards? I hear people talk about it being cheaper but that doesn't seem to be the case in Australia. Maybe the high price is only an issue in the Australian market?