Another idea: There are devices designed specifically for trimming a long film leader. I believe Leica made one called the Ablon. Searching on eBay, I see this Ablon "clone" available. I've got a similar device. I've used it a couple of times but found it awkward -- but I was using it in daylight, when ordinary scissors are easy to use for the task. Mine might also be poorly made (the pieces seemed to fit poorly), and is of a slightly different design than the one I see on eBay right now. The edges of mine aren't very sharp, so I wouldn't hesitate to use it in the dark, and it'd probably be easier to use than scissors, much less a razor blade, in the dark. In any event, the cost of an Ablon clone is only about the same as two rolls of Efke IR820, so it might be worth a try if you try trimming and loading in dim light and end up with fogging.
Of course, this won't help with loading a bottom loader once the film's trimmed, but at least it'll help you get to that point.
As PhotoJim suggests, you could also try picking up a Russian Leica clone with a removable back. I've got a FED 2 and a FED 5, both of which use LTM39 screw-mount lenses. I don't have any Leica lenses, but my understanding is that most screw-mount Leica lenses would work fine on all but the earliest FED 1s (FED 1s are bottom-loaders, anyhow, so they wouldn't be useful for your purpose). Most FEDs and Zorkis are pretty inexpensive on eBay, and of course having a spare body might be handy even aside from issues involved in loading IR film. I'm not sure when the Zorki line switched from bottom-loading to back-loading, but that switch occurred in the FED line with the FED 2. Both FEDs and Zorkis went out of production a while ago, but new FED 5s are still fairly common on eBay -- a search on "new fed" on eBay turns up a few.