For my mixte tandem, I used the strap-on mounts from department-store Bell bottle cages. They sit securely between the mixte tubes when cinched down, and they have the normal arrangement of bolt holes for mounting the cage of your choice.
I use rivnuts to make bottle bosses where there are none. Unfortunately, I don’t think they would work for you because you’d have to mount them well up on your seat tube where they’d interfere with your seat post. Although you might be able to drill holes for them into your down tube using a low-clearance angle drill . . .
As for bags, I’m really liking this Swift Coyote bag that
riverdrifter put me onto. Velo-Orange makes some nifty bag mounting loops that attach to your seat rails to emulate the bag-attachments loops that Brooks saddles have. I have a smaller burrito bag on the bars. Between the two of them I can carry a lot of stuff.
