This was derived from a comment on the game.

At least for upgrades, if you select a tower and tab to an upgrade, you can hit enter to buy the upgrade.
This works even if you don't have the money for the upgrade or have already gotten it for that tower, and multiple of the same upgrade stack.

The only problem with this is that you still spend the money, and will quickly get into irreversible negatives... so don't buy upgrades you can't afford until you have the layout you want.

I haven't been able to use it to get the hard mode badge though, as apparently I suck at it too much to get enough seed money ;_;