Not a perfect solution but a leading single quote (') is enough to prevent excel's auto-conversion.

The only catch is that when the data is read from a file, the quote will be visible until the cell is reevaluated (eg, cursor hitting return after the cursor is in the cell for editing). Might try a simple macro to touch each cell?

The "call it a text file" approach seems better but this is an alternative if a text file is inconvenient.