Ah yeah, I should have said that I've also considered that.
Except for the one problem of the apertures not being properly circular, especially the 65/8 SA having only 5-blades. I can calculate 'roughly' how big the aperture size needs to be, but mostly I'm going to be shooting Velvia with these so I can't afford to be 'roughly' correct to within a stop, I'd prefer to be within 1/6-stop or better, hence trying a TTL method...