Anyone know of a way, via lens, filter, or combination
thereof, to make the XL1 mimic the look of the GL1's
Flourite lens (in unsharp mode)?

Scott,
Could you be more specific regarding the "look" you have in mind? Is your goal to match footage from the two cameras? If so, what attributes appear different to you?

Because of the 'C.Sharpness' function in the GL1 menu, which can be turned
all the way down, the picture appears to have much softer edges and also gives
the 'illusion' of a very shallow depth of field.

I'd recommend trying a Tiffen Black Pro Mist 1/8. It provides a very mild edge softening.