Hello, Here is a link to download the manual:
https://www.tek.com/en/manual/model-dmm6500-6-1-2-digit-multimeter-user-manual
Answers to your questions:
Can you set the digitizing current function on the DMM6500 to trigger on the rising edge of a user-input threshold current? Yes – you would have to use a trigger model and set up an analog trigger, rising edge mode. Please see pg. 8-19 (pdf pg 346) in the reference manual for information.
Would this waveform trigger generate a signal on the trigger output signal on the rear panel of the DMM6500? Not by default, but you could set up an external trigger out to occur in the trigger model. See pg 8-15 (pdf pg 342) for more information
What's the time delay between the waveform trigger and the rear panel trigger output? I unfortunately do not have the answer to this and am not sure it is specified.
Can the DMM be configured to perform this function using SCPI commands? Yes but indirectly. You would have to program the trigger model to wait for the analog trigger, then assert a notify to trigger out, then begin measurements. The trigger out event would be tied to the notify block.
Can the digitized buffer be read into a workstation using SCPI command? Yes – use the TRACe:DATA? query.