This is why none of them allow (let alone encourage) hacking of surround processing or signal routing in their products.

A lot of that has to do with licensing and implementation. Usually the license forbids disclosure or modification of the algorithm, even though parts of it might be conveniently refactorable. I've often thought that the core of such algorithms remain controlled, with the bits around input and output more flexible, so, for example, a licencee can share common output level control code (and downmixing) among several decoders. But, even here, a lot can be done to open a unit up. Witness the Roku HD1000, and their requirements to not expose CSS or Macrovision control, etc.

There's nothing wrong with what you want to do, it's just that a home theatre surround processor is simply the wrong tool for the job.

If you consider the pre/pro a fixed piece of equipment, for a single purpose, I'd be inclined to agree. But, it is rapidly becoming a platform for software (and, in some cases, hardware) enhancements for new data and physical interconnect formats -- more like a general purpose computer than a specific purpose device. Particularly, when a manuacturer provides a software and/or hardware upgrade mechanism, a strong argument can be made that it is beneficial to the end user if third parties can use that mechanism to add after-market value -- rather like what Etronics did for Sherwood and Outlaw, but at a further downstream level (and not to the same degree of customization, obviously).

Other manufacturers are starting to do this (though, as you keep on harping on, not consumer A/V equipment manufacturers -- yet). Your objections to license restrictions and consumer electronics supposedly being immutable fall before the Roku HD1000 (really, the closest to a consumer A/V device that I can think of), and other, hackable consumer products - OFA remotes being another area, IIRC.

The response from the A/V hacking community has been to do what it can with what it has -- general purpose PCs, often violating matrixed audio encoding patents to decode DD and other formats in software, on fast processors. The only concession they get are A/V component-style cases, instead of the ubiquitous beige box for their HTPCs. A proper platform, with the licensed bits closed, and the right stuff done in hardware, but the rest open, would be killer.

Someone complained that the 990 does not switch HDMI, or transcode it to component analog video. I can't see how it would be impossible to develop an add-in card, to replace one of the existing ones, that does this, either by Outlaw, or a third-party.