If you look online, there are power curves posted for some trainers that plot mph vs. watts, so you can convert. Just keep in mind that your actual power output can change with tire pressure or trainer tension (depending on type). Also, unless your computer shows accurate speeds down to 10ths, you're not going to get very sensitive power estimates.

They say all models are wrong but some are useful---your power estimates are likely to be off, but at least you'll have a way to approximate power-based training.