I first laughed at this, but it's a cute idea. Not practical likely, but cute.
What occurs to me is the syringe-type two part epoxy sold on a card in hardware stores is pretty much what you want, at least dispensing. You push the plunger, it dispenses the proper amounts of resin and hardener in proportion. You just pop the cap on when done, it keeps once opened for months.
So, a stepper motor turning a screw with a static nut on the other end is all that you would need. Total hack, but it would work, and cheap... Scaling up really is just a matter of making it bigger. The mixing point is the point where trouble could get bad fast.. but there's epoxies available with setting times from less than a minute to hours. Your "syringe" can be made from PVC piping, using the proper diameter proportion to determine mixing ratio, just like the plastic-carded ones. An epoxy with a ratio of 2:1 would use one pipe "piston" full, with a 2" inside diameter, the other with a 1" inside diameter (as an example. It's the ratio of the pistons sizes to eachother that's important). If a "plunger" is run equal distance down both, it will dispense out at a 2:1 ratio. Changing mixing ratios then becomes changing in the right "piston", no calculation necessary. Your motor/pump doesn't control the mixing ratio, it just controls how much of the properly-ratio'ed epoxy you pump out. Typically, epoxies have only one ratio to be mixed at.. it's not like you vary amounts of resin and hardener to produce different types of epoxy, and their chemistries are probably unrelated enough that trying to have a Universal "A" to fit every "B" would just be silly. Only when you entirely change epoxies would you change the mixing ratios.
It may just be easy enough to use a really cheap mixing nozzle that you expect to clog, and just take off and throw away just the mixing nozzle at the end of the day. It could be made from cardboard or other relatively trash-friendly stuff I suppose...