I've done it!
I mostly "tapped into" other wires to power the various gauges, which doesn't seem like the perfect solution, but adding fuses to the fuse box and such seemed like overkill. Plus, every wire that I touched I did properly, with bullet connectors, male / female spade connectors & heat shrink wrap. For the lights I just tapped into the wire for the dashboard lights (I couldn't find one for the A/C controls). For the volt meter, I did use the old A/C connector which was on the Ignition circuit, and for the clock I tapped into the radio memory wire. I ran all the grounds in parallel to the same earth the radio was using, and this seems to be working just fine.
I finished up quite early last night, so I spent a solid hour just cleaning up the wiring, wrapping and bundling the cables in a logical manner, and everything behind the dash is so much more manageable. I'm quite proud of myself for taking on this little project, and even if I maybe should have introduced new fuses, I don't think my solution was a "hack" by any means. And as I mentioned before, I definitely feel like I know the electrical system inside and out now, so these projects won't be so daunting in the future. I'll try and grab a few pics tonight and post them up. Thanks for everyone's help. I wouldn't have started into this if it weren't for the info posted by you awesome BMC'ers.

Go Mini Go!!
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1998 Rover Mk VII
2001 Honda CBR 929RR (street)
2014 BMW S1000RR (track)
