I quickly researched this a couple of months ago and came to the conclusion that using separate accounts was the only easy-to-understand way to do it. So now I have accounts like:
Hosting Expenses
[SomeTag] Hosting Expenses
It's nice because its compatible with all the built-in reports, including graphs, though it takes a little while to configure presets for them all filtered by the right accounts.Also curious to hear if anyone knows a better way.