There are two more from QuickMill (strangely I can’t find the models on their website), which feature electronic dosing and solenoid-controlled E61 groups.
QuickMill Vetrano 2B DE (2B Rotary)
QuickMill Andreja DE (HX Vib pump)
Both link to Caffe Italia [coffeeitalia.co.uk]. They are also listed as “in stock”, suggesting the info is not totally out of date)…
So there’s an HX Vib, HX Rotary, 2B Vib, and a couple of options for 2B Rotary.
So reflecting on Mac’s original question, I bet almost all of these options could be fitted with a flow-control device, since ECM and QuickMill make them, and ACS is compatible with Lelit’s. Certainly, the manufacturers could be expected to answer questions of compatibility of their own parts.