Thanks for your responses so far!
We did have a look at Vega Strike briefly the other day, and whilst they are sporting an 'Ogre3D' badge, a glance through their code suggests that they abandoned it some time ago.
I can also attest to not wanting to use Ogre - it wasn't pretty last time I tried

It's tailored more to building an engine around, not just using as a graphics front end (that was my experience, at least, and that may be entirely wrong).
@Dragon: It's not going to be done in-engine. Whilst the needs of the engine will be kept in mind, the majority of development will be done separately in order to ignore any limitations introduced by the engine. (which will need to be overcome at some point!). We'll need to get feedback at some point though!
@Raven2001: The issue with the engine is that it is incredibly 'coupled'. There is no real separation between components. This is needed in a modern engine and for coder sanity. Changing something at the moment is a nightmare because you don't know what else it changes! The collision detection code hasn't been touched because coders that venture there don't usually come back

It's not that we're focussing on the graphics engine, it's that we're focussing on removing the massive coupling to the graphics engine, which will likely result in better management of the scene.
'Technical Artist' is a term for someone who is able to do both art and coding and knows how they affect each other in a particular engine.
@karajorma: Collision detection code is.... ugly.
@Solatar: If you can read, we'll probably need you. It's amazing how often someone reading some documentation or planning says something like 'there's nowhere for the trucks to turn around' (true story from an engineering project in Australia).
@FreeSpaceFreak: We need people who use the engine (i.e. modders and coders) to tell us what they want and expect. We're planning to do this separately so that we can evaluate any existing options and/or design our own without having to consider any limitations of the engine.
@ChronoReverse: Technically, we have different requirements to MMO (or whatever you call those Age of Empires) style games, and that seems to be what Ogre is designed for. Legally, we have issues with licences because we're effectively 'no commercial use', which is incompatible with GPL and we'd need to talk to a lawyery kind of person if the software didn't allow us to do this (we have code in one case that a project developer has licenced separately to us, and we also have code that is under NDA (trackir) )
@blackhole: This project isn't going to be a coding project for a little while, it won't become a serious coding project until probably way after 3.6.12. You've added a pile of very useful questions. It's more likely to be a C++ API than a C API though. Hery did mention a desire to play with SM3.0 for particle systems.
If you're able to give us any time at all it would be great!
Thanks for your responses everyone! We'll keep you posted on what is going on (likely just organising style things for a while). Please keep an eye out for friends/family who do coding or have expertise in art or game design.