I know nothing about tax law, so this is an important disclaimer. I won't end up trying this.
To spell out the idea, one could have their Roth IRA and their investment account at separate institutions. The Roth IRA broker would simply see a loss (gain) and the investment account broker would simply see a gain (loss). Each institutions reporting these changes to the IRS is the benefit of the finagling.
Yes, and the IRS would see both transactions under your tax identification number (for an individual, typically the social security number). The IRS is going to have an opinion about what that transaction means for your IRA account, and if your tax return is not consistent with that opinion, the IRS will expect an explanation.
Disclaimer to any reader: I am not your lawyer. I am not a tax lawyer. I didn't do any legal research. Don't rely on my opinion for any reason. Definitely don't rely on this opinion to try and pay less taxes. If this is a real issue for you, hire someone to do the research, or do it yourself.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.