Dan,
The prototype shirt you have on hand - is the same exact shirt on order except for the fit?
If it is (weight, color, wash etc), then maybe you could do a couple of things
1. get your hands on as many of those as you can, same size but different colors probably would be the best in your case, since you have all this artwork to show. Does your t-shirt manufacturer offer samples? Even at charge? If so, buy a few, buy a pack if they offer one, it's worth to buy those and pay the shipping etc. I think it would highly increase your chances of success with the buyers. If not - demand it, with 600 minimum I think you deserve to have a sample.
Then I'd follow Lou's advice. Either do some transfers at home, or find a local contractor who'll do a small volume order for you. I understand that transfers may not be your planned method for future or the finished product but I think it's a better idea than presenting a blank and then artwork separately in digital format.
2. then I'd work with the manufacturer on having them make you at least one sample of the finished product to show the particular fit/size you and your partner are concentrating on. I know these can be costly, but again it's worth it.
Just in general, I'd do my best on trying to get the prototype(s) as close as possible to the actual thing. The further it is, the higher the risk of getting rejected, or even worse - getting the order, than realizing the actual product is slightly different from the samples originally shown to the buyer, get in trouble etc etc
Good luck!