My $0.02 on this:
All you really need are some "known" fixed points for voltage, resistance, and temperature. So, a 10V reference, a 10K resistor, and a temperature probe. With just these 3 things, everything else can be derived if needed.
Each of these should be designed to be: Relatively low cost (compared to commercial standards), rugged enough to be roughly handled without losing calibration, small enough to be shipped in the lowest cost priority mail package, robust enough to be used by inexperienced people without damage, and stable enough to verify up to an 8-1/2 digit meter (or even calibrate a 3458A if you are lucky enough to own one of those).
This suggests: An LTZ1000-based 10V voltage reference, a quad-VHP100 based 10K resistor (similar to Lymex's design), and a PT100 based temperature probe.
The voltage and resistance standards should have an uncertainty [k=3] of no more than 0.5ppm, and the temperature probe should have an uncertainty [k=3] of no more than 0.01oC.
If a person needs to have more voltage or resistance cardinal points, they would be responsible to create them on their own, using the supplied references as standards.
Obviously, calibrating these standards would be expensive. Someone with an SR104 could get that calibrated (about $400 plus shipping and insurance, maybe $800 total), and that should be used to calibrate several in-house 10K standards, as well as the 10K traveling standard(s). Someone with a Fluke 732 could get that calibrated (about $500 plus shipping and insurance-- maybe $1000 total), which should immediately be used to calibrate several in-house 10V voltage standards, as well as the 10V traveling standard(s). Someone with a set of fixed-point cells could calibrate the temperature probe(s). Costs for these calibrations would be shared across the calibration club, and shipping of the traveling standards would be paid (in each direction) by each end user for each use. Calibration of the traveling standards against lab standards before shipment and after return would be done on a volunteer basis by the central hub (which could be one hub that handles all standards, or as many as three hubs that only handle one type of standard).
There could also be a down-graded kit for people with 5.5-digit (or less) equipment, consisting of an LM399-based 10V reference, a single VHP100-based 10K resistor, and an 100K NTC-based thermistor probe. This kit would also have a slightly downgraded uncertainty for each standard-- say no more than 3ppm for 10V and 10K, and <= 0.1oC for the temperature probe. This would have a lesser participation cost than the higher level standards.
This is how I see it as the only alternative for a serious calibration check among club members-- anything else is just "playing", and provides nearly zero real value other than entertainment.