I can add a Cwejman VCO-2RM and Cyndustries ZO to your list (C1 - C6), with the caveat that the Zero wobbles a bit in the lower registers and, once you start applying FM indexes in excess of 100%, pitch becomes a relative issue
I also got a Cwejman MMF-1 filter to track perfectly from C1 - C6.
Doepfer's A-105 is a little less stable (+/- 1-2 cents) over the same span, but easily close enough for rock 'n' roll.
Malekko/ Wiard Borg didn't fare as well: +/- 15 cents, but I didn't expect it to.
Two wild cards:
Bananalogue/ Serge VCS, C1 - C5, up to 6 cents off, but hey!
Plan B Model 10 EG, C2 - C4, +/- 20-40 cents. Also surprising.
The tricky part here is figuring how much Timebase modulation to apply.
With the VCOs and filters that are calibrated for 1V/O maybe these results shouldn't be too surprising.
I guess the real issue is that there are still only two (three if you count the RME's phones out) compatible audio interfaces ...