Michael, a couple of issues.
1. the Adventurer by steripen requires being able to see the LED chamber light -- with the soft-sided nalgene, the plastic is too foggy to see the LED Chamber light (at least for my own eyes) -- the hard plastic containers, one can see the LED Chamber light. the new (but about 2 oz heavier and more expensive) steripen that came out this year, has a LED screen and perhaps could be used with the soft sided nalgene container just well, but for me, the adventurer--I did not like with the soft sided nalgene container.
2. as for the Platys widemouth bags, I forget what Steripen had to say about that type of plastic--I thought they had a concern about this, but I forget the details. I'll try and dig around and look up the info.
Bottom line, I think the latest Steripen would work well with your method and the soft sided Nalgene bottle, and if one has far better eyes than mine, the Steripen Adventurer might work well too (you need to be able to see the LED Chamber light in the outdoor daylight situation one encounters outside in the wild--not what one encounters inside a house experimenting around).