I looked in the rulebook. Couldn't find a prohibition, did I miss it?
So, legal or not?
As noted in post #7 from the WR 2012 TG meeting minutes, the LEGALity of elastic for ammo loops was a reiteration of an RO Committee ruling that was made back in 2009 (or thereabouts) based on the fact that elastic is a "period" material (shoes, suspenders & other articles of clothing).
Those minutes were posted on the TG Wire for the Territorial Governors to pass on to their constituents.