Hmmm... I step and sail most of the time alone. In general I follow the 12-step program described by H2814D.

With one change... most of the Hobie 16's today are set up in a way so the mast is raked back significantly. Due to this mast rake it will be difficult to find an area to park boat and car on an a steep gradient - steep enough for the mast to point up vertical or even a bit past vertical. At least I do not find these places. I attach a rope to the end of the forestay loop it around a piece of hardware on the trailer and place it on the trampoline in an area where I can reach it while supporting the mast in its vertical position. The rope allows me to temporarily secure the mast while I get of the trampoline in order to pin the forestay.

Jim... you stated you were concerned that you might bend the mast while pining the mast to the crossbar due to the mast hanging unsupported over the rear crossbar - Do not worry that will not bend your mast - It will need a lot more force than that... a lot more!!!
Also... you seem to come up with some ideas to substitute the lack of trap wire on your 16. I am sure the way you are planning to set your cat up will help you step the mast, at the same time I would suggest to get the trap wires and if necessary the piece of hardware to attach it to the mast. Stepping the mast of a Hobie 16 with out the support of trap wires is one thing (I do it all the time) Sailing a Hobie 16 without a trap is another. I did not even know that there are old Hobie 16's out there that came without at least one pair of wires. Upgrade to trap wires - use them for the mast stepping process if you feel that is safer - use them for sailing the cat because it is faster and more exiting and often necessary to handle the large sail area.
Patrick