"I'm sure that if a way could be devised to switch in the capacitors relative to load then one could build a DIY 'digital' RPC."
Dropping back to basics, a phase converter, rotary or digital, is providing a source/sink of volt-amperes reactive (VARs), in response to the load conditions, transient and steady state.
A rotary is usually stuck with a "one size fits all" solution, and the use of real capacitors, whereas a digital can be a "best fit" solution, with the use of (digitally) simulated capacitors.
Implicit in the digital is the ability to compensate for transient load changes, which occur within a cycle, as well as long term load changes, such as powering on a machine, with equal facility.
Also, a digital can present a unity power factor load to its source, whereas a rotary almost always presents a lower if not very low power factor, usually lagging, to its source.
Strategies for distributing the source of leading VARs, the capacitors, have been discussed here previously.
Essentially, this means adding the required VARs, that is, the capacitors, on the load side of each machine's main disconnect.
The makers of Phase Perfect attempted to market a lower cost digital, which they dubbed "Phase Simple", but it was quickly withdrawn from the market.
Apparently, a digital of the type hypothesized by the earlier writer, namely one which is conceptually a fixed frequency VFD, was not a success.
If you have any expensive CNC equipment, a digital, such as a Phase Perfect, may be the best solution.
Otherwise, a rotary, even a DIY one, may be the most cost effective solution.