Edit code/word with an alpha field

I have a DSPF field (5,0) that I have to distinguish between a blank field and a zero value ("because it HAS to work just like the old program"). So I define the screen field as a 5-character field with Keyboard Shift of "D" and Check(RB).

So far, so good? Oh, no.

They need to enter negative numbers into it. No problem - I change it to a Keyboard Shift of "M".

Everything's kosher, right? Nope.

When they key in a number and press Field-minus it shows the last digit as a letter. I understand why; they don't. Major meltdown #1. Then they press enter and get an error (meltdown #2 - ok, this one I can buy).

So I show them that they can enter a regular minus sign (next to the zero at the top of the keyboard) after the number and press Enter and the program handles it properly so, everything's kosher, right?

No.

They want to use Field-minus, and have the negative "number" show as a number (no trailing letter).

I can take care of everything except having the "number" show properly. Is it possible to use some kind of edit word/code with an alpha field? And would that help anyway? They want it to show properly before they press Enter.

My apologies for rambling. I've tried defining the field as numeric and doing an alpha overlay to check for "zero-vs-blank". Doesn't work; if the numeric field is zero, the alpha overlay is filled with zeroes.

Any ideas?