Yeah, I would be surprised if a calculator program supported that. My advice would be to settle for something a little less, like having an easily accessible shortcut or hotkey to start the calc.

I believe you could make a simple calculator with a skinbox, a label, and a really complicated script. A scientific calculator though, that would be extremely difficult.

It also might be possible to trick a regular calc app into sticking to the desktop, but you'd need a custom module or something.