Have the sprite for the glass helmet bit have the face on it but made to look like it's under transparent blue glass?
Could always have it set to getshitbymos = 0.

Or you know, have the glass gib when it falls off and the helmet bit just be a bigger gib.

Or give the helmet two frames, first with the face and second without, and no .ini animation properties, and apply a script that sets the frame to the second, which wouldn't be active until the helmet were knocked off.

That wouldn't end up looking very pretty, just saying.