We’re not experts, but we fancy ourselves fairly knowledgeable about hair. That being said, we’re not sure there’s anything available that would be as temporary as you want. A wig is going to be your best bet, because it won’t ruin your hair color and you can use it multiple times.
If you’re going to a con that runs multiple days, you could wear all of your red headed costumes and then the night before your fem!Lock, you could use a semi-perm dye. The dye will almost completely fade within a few weeks.
We’ve also read about using india ink and brown shades from brands like Manic Panic and Special Effects to stain your hair, but we’ve never tried those.
There are ways to strip your hair of dye so that it’ll come out faster, (like mixing vitamin C powder with shampoo and applying it to wet hair for a few hours until the color comes out *hint hint*) but we would like to go on record and say we don’t suggest it.