Stephen:

I cannot directly answer your question, but perhaps someone at Light
Impressions could tell you more about the safety (or dangers) of
encapsulation.  They are a leading supplier of archival materials and
do offer mylar polyester film for encapsulation.

Here is their information:

Light Impressions
P.O. Box 940
Rochester, NY  14603-0940
1-800-828-6216 (technical assistance, M-F, 9-5 E.T.)
www.lightimpressionsdirect.com

Good luck,

Christy Sinksen
Assistant Registrar
The Georgia Museum of Art