Ok I have some more experimenting to do before I make a video of it (I feel like there’s a less kludgy way) but let me describe how I set up a sort of auto-sample.
On Magneto:
-Sample mode, make sure tap light is off
-only Head 4 on to start
-speed at noon
-patch your keyboard or sequencer’s V/Oct out to Speed CV in, and its Gate out to Play CV In
-patch a Trigger out (in my case, from the FH-2) to the Tap input
In Ableton:
-Track 1 has an unlooped audio clip of what I want to sample, routed from an output of my interface to Magneto’s audio in. I started with a single piano C just to verify this all works.
-Track 2 has a MIDI clip routed to the FH-2 to trigger Tap. The clip is unlooped and one bar longer than the audio on track 1. In that clip, there’s a trigger on 1.1, which will tell Magneto to start recording (Tap flashing) as the audio clip plays. Then there’s a second trigger on the first beat of that extra bar, which will tell Magneto to stop recording (Tap lit).
-To sample, launch the row with both those clips. Magneto should have that exact length of audio loaded into it, playable from a keyboard or sequencer!
It does have a bit of a built-in glide (inertia of the “motor” changing speeds). The Speed knob still works too, so you can use it for fun scratching/pitch bending tricks. Noon is original speed/speed, full CW for an double-speed/octave up, full CCW is half-speed/octave down. For bonus fun, with a sequence running, try sending triggers into Reverse 
This module is so fun! Next step is to get a handle on the looper and get that synced up too!
