Does Mantra support OIIO? Or are we still tied to the awesome (but proprietary) rat format?

Nope, it doesn't. I think that would require dropping Houdini's own image dso to avoid library clashing.

I imagine Mantra texture subsystem could support natively tiled EXR as a minimal consensus. Last time I've checked any non-RAT file is silently converted to RAT on load. RAT texturing used to be years ahead of many renderers' one, it would be a pity to drop it.

btw, does OIIO cache localize textures per tile when they drop-off the RAM quota?