I wouldn't worry one bit about using the Envirotex on pieces that will be outside. I've used it on emblems and grill badges for a handful of years no with no adverse effects from the weather. I even put a thin even coat of it over the lenses on some driving/fog lamps to fill the pits in the glass from road rash, still looks bright, clear, terrific.
http://img.photobucket.com/albums/v136/TeamEvil/fog2.jpg
http://img.photobucket.com/albums/v136/TeamEvil/fog1.jpg
Since this product is used predominantly over wood, I think that this is where the caution is directed. Since wood breathes differently outside than it does inside, and absorbs moisture, swells and contracts, and the Envirotex hardens very ridged, almost like glass, I imagine that it would crack and peel on outside decks, deck chairs, boats, and such. I'm TOTALLY thinking that the caution is meant to keep folks from using the stuff in place of weather-proof urethane or wood finishes meant for the outdoors.
Scott, I can help you out with those emblems, but it'll be a while, OK? Right now I'm pretty much doing the same sort of detail re-furbishing on the bits and pieces for the Fiat 600. I picked up some wonderful original Abarth stuff, all of which needs to be reconditioned. Some rare wheel emblems and genuine Veglia accessory gauges as well. Dash knobs, horn button, shift knob, etc. all of the "smalls" need attention to look factory fresh.
After the Fiat is all set, I'll let you know on your wheel caps.
This crest was re-painted and hit with Envirotex.
http://img.photobucket.com/albums/v136/TeamEvil/button1.jpg
You needn't worry about my expertise, I have a serious knack for this shit . . .
Oh, in addition, the Envirotex is AMAZING on wooden steering wheels, but you have to keep rotating the wheel on different axis for a couple of hours so that it sets up a bit. It makes for a VERY long (and tiring) night of television . . . but it looks SENSATIONAL ! ! ! Like nothing else.