Hi John,
Obviously passing oddy testing is pretty much of a pre-requisite for a replacement for Pliacre (more recently sold as "Phillyseal").
At places like the Getty (in a seismic zone) in addition to oddy testing due to the weight of the objects that they are dealing (and related amplified degrees of stress) another issue to be addressed is compression testing (just like concrete or anything else) where the functional limits of units of pressure per square inch needs to be established. There was a document published last year in the WAAC newsletter by the antiquties mountmakers at the Getty Villa that may contain some of these specifics. I have asked for permission to post the article. Hopefully I will be able to pass it on soon. Unfortunately from everything I hear although there are alternatives available I haven't heard of any that have the exact working characteristics (some are stiffer, some are "goopier"). As a result some adjustments in terms of working methodolgy may just be inevitable.
Ashley