Lennie, glycerin tanned skins are not actually tanned but cured. I have used these before but the ones I used had been glycerin tanned about 20 years before so the glycerin had disappeared.
Tanned skins, like other leathers, will soak up moisture like a sponge so, IMO not good for bow backings.
The first Community Bow was backed with tanned python skin so it does work but there are better options.