Hi MarkSince you haven't had any replies yet, I thought I'd come in (in my state of newness) and offer what I can. We have the OA3 at Harrogate HR - and I have the book kicking around too - and I'm pretty sure you can download it from Sonifex too - it's a single book for the majority of the RedBox range.
The OA3 has an indication line to show which studio is on-air, which I'm sure you could use to go into the Advantech - though I believe, from memory (and I may be wrong) it's the same line which drives the 'Accept' light (which is steady on, in the on-air studio). As long as the Advantech can handle this voltage (which again from memory is around 15VDC at a fart of mA) there's no reason why that shouldn't switch Myriad as you wish.
How to actually configure the Myriad side of things - well, that's beyond me, as I'm *VERY* much a Myriad Novice at the moment.
If you wanted to go the 'other way' and have Myriad control the OA3, I think you'd be going up a steeper hill, as you'd have to define logic to control the 'Offer' and 'Accept' buttons - especially if you use the 'Sustain' option of the OA3 (press Offer and hold Accept to send on-air to 'Studio 3')...
I'll finish this post here, but see if I can find the link to the manual, and get it up here for you... I hope my ramblings have been vaguely useful.