send a series of sysex at a keystroke?

I'm afraid I can't see a way to do that (that doesn't necessarily mean there isn't any), at least not the way you think - there are no direct actions to send sysex, MIDI controlling ReaControlMIDI to trigger the sysex send button isn't possible either since there's no envelope for it. I couldn't find one quickly but maybe there's a 3rd-party plugin that can store and send sysex data with a button, the button would most likely show up in the envelope menu. What would work is when you're playing with playback running, to an existing song structure - I would insert sysex data with the needed pauses in an extra item (switch a CC lane to sysex in the ME) and copy that item to all places where the stuff should be send in the coarse of the song. As for the second part of the question, you should be aware that Reaper doesn't have 0-latency MIDI-thru, this is linked to the audio latency. If you're not dealing with audio at the same time you can probably set that to ridiculously low buffer settings, otherwise there's MIDI latency = audio latency, so you want to keep that as low as possible (on modern systems going down to 64 samples is not unrealistic anymore, that's almost on the "long cable" level).

you can get sysex to be spat out on demand using the fact that sysex gets sent when you change/select the midi channel in reacontrol midi. using that i made a macro and template that adjusted the midi channel back and forth (as it was made a track knob and there are actions to adjust those) to send the sysex, and that then could be assigned to buttons/toolbars etc.! i did it to be able to change a bcf2000 preset from within reaper. here: the tricky bit was getting a macro to adjust the midi channel reliably. basically - make the the midi channel select on reacontrolmidi a track knob, then make a macro with these actions in: adjust trak FX parameter 01 (midi cc only) action:modify midi cc/mousewheel: 0.5x adjust track fx parameter 01 (midi cc only) using that then should spit out on sysex in reacontrol midi. further instances and track knobs can be added etc.

Wow, that's nifty, BenK.