not sure why this is a problem. what particularly are you thinking of? i'd like to learn something from you.
right now, if i'm us UV-C, i 1) fill the bottle; 2) then use a small scrap of pack towel for drying the outside & threads - this is the sole purpose of this piece of towel in my kit; 3) use the Steri-pen; 4) put the bottle top on.
my bottle caps seal tight & so water on the outside from filling (i.e. the threads) does not mix with water in the bottle.
proof of this is that if i fill a bottle at home. dry off the outside. & place the bottle upside down on the kitchen counter-top, 24hrs later no water has leaked out of my bottle onto the counter top. if the pressure of the column of water in the inverted bottle can't force the water out, then water on the threads can't really get in.
however, it's a good idea to dry the threads of both the bottle & cap off first as added insurance since don't want to chance the slim possibility of the screwing motion of replacing the cap, just b/f being fully tight, of acting as a pump & forcing a small amt (a couple of drops???) of water off the threads & into the bottle.
having said that, i still use Aqua Mira as my primary source of water purification since it's lighter.
haven't used UV-C with a bladder yet, but thinking "out load", so to speak, & off the top of my head w/o giving it much thought, what about this:
if using a Platy bladder,
1) simply remove the drink tube &...
2) put a closure cap on it (if bladder is a Big-Zip; skip this step if just using a hoser)
3) fill the bladder;
4) wipe the threads/zip-closure with scrap of pack towel,
5) use UV-C
6) put tube back on the Hoser, or...
6a) zip closed if filling a Big-Zip,
6b) remove closure cap and
6c) then put tube back on.
when you get a chance, please enlighten me as to what you feel i am doing wrong.