Force any printer to convert all colors to Grayscale automatically? I've got a color inkjet printer, and I only have black ink left. 

I want my printer to automatically convert all colors printed to a grayscale equivalent.

I'd like a solution that works with any printer, not just my specific printer.

Is this possible using a virtual printer or something like that?

Note that as stated in the title, I'd love to be able to do this automatically, so I don't have to select options in a print dialog every time.
A "one-size-fits-all" approach may not be possible, as different printers have different capabilities and driver controls for specifying Black ink only. 

You could try the Color Matching pane of the print dialog: select ColorSync, and select a Greyscale or Black &amp; White color profile. However, even if you do this, some printers may still try to print this as a '4-color Black', to spread the ink usage across all the cartridges.

Your best option is to determine what controls your printer has (if any) for printing to Black ink only: then select them in the print dialog and save as a Print Preset.