The super insulation and small cabin size should keep the heating demand pretty low.  
Why not put a wood burner in each cabin and heat with that?  Zero net carbon, and it seems like people using a cabin might like the idea of a wood burner?

If it was done carefully, you might heat water with a coil in the wood burner -- this has some steam explosion safety hazards, so the plumbing needs to be done with care.

On the sewage side.
Maybe consider composting toilets?  Again, this might be something people renting cabins might like to try?  No septic tank or septic tank pollution issues.
Some of them don't work well, so its important to do your homework.

We stayed in one set of cabins up at Lake O'Hara in Alberta that had actually installed an air (vacuum) toilet system.  They have a trunk line that runs past each cabin.  The lines are mostly above ground, and I suppose that with this type of toilet, there is no danger of freezing.  The system seemed to work quite well -- gives a very satisfying whooosh every time you flush.  Not sure what the power or cost implications are.

The solar water heating on the central laundry should work well, and a regular carpenter and plumber should be able to build a bigger version of the $1K type system with no problems -- its all very straightforward carpentry and plumbing.

I suppose it would be possible to keep the toilet chamber heated somehow, as with some of that hot water, but, again, in a commercial situation there will be, trust me, some unhappy "campers." And, the heating system to do so would, in my opinion, add another level of technology, upkeep, and hassle to the system.

There are Incinolets, the electric toilets that burn the stuff, and the propane ones, likewise. Of course, you would need more electricity or propane to do that. I looked into both; the electric ones use far more electricity than one would want to provide with PV or wind [except at the exact time the wind was blowing sufficiently], and there was no way I was gonna buy propane or any other such fuel to run my toilet. If I recall [and it was some years ago that I looked into this], the co. selling propane-fired toilets suggested that for a 2-person cabin, one could probably expect about 30 gal. per month propane useage. I have no experience to tell me whether that is accurate.

Researched this stuff while designing a Net Zero Energy Home. Not built.

PEX piping is freeze damage resistant and can expand and contract as water freezes and thaws within the tubing. No tubing material is freeze-break proof, however, and PEX should be installed using the same locally-prescribed insulation requirements to prevent freezing of any plumbing system.
http://www.ppfahome.org/pex/faqpex.html

Rain water collection, Skip the well.
http://www.arcsa.org/
http://www.harvesth2o.com/

Gray water - http://www.oasisdesign.net/index.htm
Reduces load on septic system. Not costly to implement.

Aerobic Septic System (ATU) requires air pump. Water can be discharged to surface of land. Once saw a dual system, first stage standard (anaerobic) septic followed by Aerobic Septic System, claimed could discharge water into lake or stream. Many designs available, MUCH researching required to get something your configurable with. Your system requires a pump (up hill), may as well send clean water down hill. Cost is 2x+. Use wind powered air compressor?
Newer ATU have a small anaerobic tank (often called sludge tank), because of its small size requires pumping more frequently than a normal septic system requires.
http://www.inspect-ny.com/septic/altaerobic.htm

Wind induced ventilation. Tightly sealed homes require outside ventilation, use the wind.