Most G-Shock adapters are designed for 22 or 24mm one-piece nylon straps. (Casio's original design was for a 23mm nylon/neoprene/velcro strap.)
From what I can tell from their website, Isofrane straps are the classic, "normal" two-piece design that use a springbar at each end to anchor the strap.
It MIGHT be possible to modify Keoni's Shapeways adapters to accept an "extra" springbar in order to use a two piece strap, but I wouldn't really recommend it for either the plastic or metal versions, since they weren't designed to work that way. (If someone wanted to attempt it anyhow, the metal versions
should be stronger in theory, but also a harder material in which to drill new springbar holes.)
http://www.shapeways.com/shops/keoni?gclid=COyJ0Y_b3c8CFQwTgQodRDgDgg
Because Casio's original adapters are made of a softer plastic, it probably wouldn't be a good idea to even attempt such a modification since there's no way to tell how long the plastic would last before tearing. I haven't personally handled Jays and Kay's adapters, but since they're improved versions of Casio's design they're probably constructed of a similar non-rigid material.
The best bet for using a two piece strap is probably Suppa's metal adapters:
Suppa G Shock Adapters Replacement Straps
While
"(t)hese adapters were designed with Zulu straps in mind" Suppa even sells their OWN two piece nylon strap to be used along with those same adapters.
Or, as revlimiter pointed out already, it might be possible to just cut "notches" in the strap to make it fit the 16mm Casio lugs.