I have a 100w merc. Do I put a 100w incandescent in series with the merc?

At first it would not be "selfballasted" lamp, but lamp with an incandescent ballast.
But at second, simple series connection work only on 230V mains (then the 230V/250..300W lamp would be about the correct ballast for ~100W merc; you would have to check the operating current after warmup, it should be 0.95Arms for 100W MV).

120V is not sufficient to cold-strike the probe start lamp, it need an electrode preheat. And that mean totally different arctube design (instead of starting probe there should be preheat filaments on each end and a bi-metal "starter" switch), what you would find only in real 120V SBMV's...

Would a superimposed HPS ignitor help ?

I have 240v threw a step up transformer so voltage is not a problem. So a 100w merc and a 300w incandescent in series will work?

No, you need to keep quite high current at the elevated voltage to transfer from glow to arc discharge (i.e. heat up electrodes). So the bulb may show tiny flashes, but it would not develop an arc...

What can I use for the high current needed?

You need higher OCV, at least 200V, so either use 240V outlet (violate the code) or an (auto)transformer (250VA output). Then you connect the lamp in series with the incandescent of required wattage (to reach the required 0.95A current; need fine tuning).
But that mean, then the transformer is part of the ballast, so the setup loose it's beauty of lightweight assembly and still it would be as inefficient as SBMV's are.

Can you light in on FL rapid start ballast ? (using just one wire from each lamp end and leaving the other one unconnected)

You can (provided you have a ballast with correct current), but then you end up with HX ballast, so no tungsten filament anymore...