Do you have to see the signal right after the trigger?
Through SCPI you may be able to wait until your initial trigger condition, then sent a series of commands to switch timebase and record a single shot.
Otherwise, I think the conventional way with a modern DSO is to rely on the long memory depth and just brute force it. Zoom out as far as you can with seeing the required detail level of the trigger, move that as early as possible in the memory capture, and hope you have enough memory to see the end of your signal. Another option could be using a scope on a fast setting as your trigger device, then running a trigger output from it into a much slower timebase scope or other digitizer, so you use the trigger scope to find the condition and then just turn on the other recorder with the signal. This may be possible in some architectures, but it's effectively like a dual timebase solution and I don't think it's that commonly supported in DSOs.