A V/UHF handheld (e.g. VX-5 or VX-1) is unlikely to do you much good "hundreds of miles from civilization." You need HF for that; I'd specifically look at NVIS on 80 &/or 40M. That said, it's my understanding there are a *few* wilderness spots where there are linked repeaters covering a considerable area (I seem recall that the area around Denali has multiple linked repeaters on mountaintops that provide pretty significant coverage, for example). But that is something you'd need to research for each trip.
A 40M CW (Morse-code-only) QRP (under 5W output) rig can be built in an Altoids tin (e.g., FOXX-3 from Kanga USA). With a lightweight wire dipole, you can keep weight manageable, but you'd have trouble seting up the antenna in the amputate-your-own-hand scenario. Using NVIS, you can get the kind of range you need. [NVIS = Near Vertical Incidence Skywave, a propagation mode generally used on 80 & 40 meter Amateur bands to obtain reliable communications out to a few/several hundred miles, depending on conditions, power levels, antenna systems, etc.]
Another option would be to find something like a used Mizuho HF handheld. They were made for the non-WARC bands (80,40,20,15,10M) and have SSB (voice) capability. They are still QRP, and you'll still need longish wire antennas. Weight will be a more serious issue here, but they've a fair amount more capability also.
The next step up is something like the Yaesu FT-817. You've completely blown your weight budget at this level, though. It has 160M through UHF in one box. Antenna requirements range from the wire types mentioned earlier to typical handheld "rubber duckies" on V/UHF for limited range. The SGC SG-2020 is in roughly the same class, but has 20W output, is more bulky, more finicky about battery voltage, and doesn't include V/UHF.
Some folks will tell you QRP isn't sufficient for this application. While my personal choice certainly would not be to rely solely on HF QRP in a life-or-death situation, it's surely better than nothing. Getting beyond QRP, given your stringent weight budget, simply isn't going to happen, and in New Zealand, the mountain rescue service rents out lightweight 1W radios to untrained backpackers all the time as survival equipment.
Personally, I don't see the point in taking a V/UHF HT on a backpacking trip unless you know *for certain* there is a good repeater covering your intended trekking area. If the goal of the trip is backpacking and I wanted a radio along to play with and maybe provide some EmComm comfort, I'd take the 40M CW rig. If the goal is playing radio from a neat location, I'd take the FT-817 (or maybe the SG-2020). Some lightweight antenna wire (26 ga. Teflon-coated) and a small, lightweight manual tuner (e.g. the NorCal QRP Club's BLT) would be great for multi-band operation.
Hope this helps.
Steve AB8JC