I have a stage race in Jan where the prologue is a short (15ish minute) time trial. The course is a flat out and back. It has to be ridden on the same bike you will ride in the rest of the stages, no time trial bike are allowed. Does anyone have any tips/hacks for riding a time trial on a road bike? I have been practicing riding in the aero hoods position once a week, and I can now confidently hold that position for the needed duration at around about the power level I will need to hold. I will also be using a time trial helmet and skinsuit, and a disc cover for the back wheel. Would marginal things like removing the front derailleur and/or brake make any measurable difference? Its pretty hard to get away in the remaining stages so every second from the time trial is quite important at the end of the weekend. I plan on having a couple of practices on the actual course before the race to dial in my pacing and equipment.
If I learned anything from the Tour de France this year, get as long a computer mount as you can and use it as a makeshift aerobar 
I’d say you’ve probably got the right idea, work on your position and take care to be as aero as you can.
Actually good point about the computer mount, I have been thinking I want it as far forward as possible so I can keep my head up rather than looking down (for power data). Are there any solutions out there for a out-in-front extension?
For a 15 min TT don’t worry about your power data. Just go all in! I wouldn’t remove parts from the bike. You risk an unnecessary mechanical if anything is broken or not able to be fitted easily again. If you have a long tail on your TT helmet, keep it out of the wind. Aero shoe covers might be another marginal gain.
I’d be also thinking carefully if the disc cover is really going to make a difference - though I’ve never looked into these.
Something often overlooked is your number. If you’ve got a number pocket on the back of your skinsuit, that’s perfect. If not, make sure it’s pinned on well, and not flapping about in the wind acting as a sail to slow you down!
I also wouldn’t bother messing with the bike too much - would you even be allowed to start without a front brake? No bottles or bottle cages though that’s simple and measurable.
Otherwise your energy is probably better spent dialling in the small things. On top of @AndyGajdas comments - I’d make sure I have a very good warmup beforehand. Tyre choice - see https://www.bicyclerollingresistance.com for data. Could also get a wax treated chain for the event - otherwise even a new clean one would be a good idea.
Regarding aero socks, I’ve noticed they’re pretty popular at the moment - but they seem to be pretty variable in effectiveness, and as I understand it hard to test. https://www.performancewatts.co.uk/aerodynamic-testing/ have published their data - they’ve concluded that they’re faster from 50kph+. Others seem to be faster at speeds below that - dunno about you but I’d struggle to do 50k average. A quick google and you can find their competitors from google.
Will they let you use a disc cover if they are not allowing TT bikes? Other road bike only TTs I’ve come across have some wheel restrictions, such as no deeper than 90mm depth and at least 12 spokes, and no full on TT helmets with visors, and also no ghost aero bar position (forearms on bars).
Don’t bother removing front derailleur or going single ring, unless you’re happy to spend the evening of the night before and the evening of the TT to change things back and forth.
Most of the easy aero savings you should probably be doing regardless of the TT, such as aero socks, optimising aero position on the aero hoods (turning them in seems pretty popular at the moment). Good quality lube/waxed chain you should have anyway.
Depending on how much you can take with you, deeper section wheels will help, if you have a skin suit you should probably be wearing it for the road racing anyway (unless it is fully time trial specific with no rear pockets etc or ability to pee) .
If they allow clipon bars, this will be the biggest saving, though if they don’t allow this, they are unlikely to allow a disc cover either.