Curt - I have the exact same Mocal sandwich plate with thermostat that you do, and an Accusump plumbed into the return
oil line from the cooler. I have a (big and expensive) non return valve also in the system, effectively before the accusump take off lfrom return side of the cooler. All lines are run in -12 aeroquip hose.
It all works fine - the accusump refills with
oil after preoiling the engine, even though the
oil thermostat has not yet opened.
The reason being that the oil cooler is still pressurised (above atmospheric) by the oil system in that condition (because it can still "see" the post-filter side of the oil system). It is the flow that is diverted by the thermostat, it does not stop the oil cooler from seeing oil pressure at any time. If you don't believe me - try cracking open a cooler union when the engine is cold but running!! (have plenty of rags ready).
The non-return valve is essential to stop the accusump oil reserve from running back into the sump via the oil pump (pre-oiling condition).
Like you, I mainly fitted the accusump to protect the engine during my 2 or 3 times a year open track sessions, but I also really like to be able to pre-oil the engine before starting it up, especially when it has been sat for a couple of weeks after being shut down from scalding hot!
BTW - take a very close look at that Mocal take off plate - I had to do some serious grinding on mine to open up the oil passages - especially around the internal porting for the thermostat, and near the hose take off ports.
HTH
Wilf