If you're willing to go that tight, isn't the L-2500 neck already wide enough? If so, it's just a matter of a new nut, drilling and installing a new tuner, and replacing the bridge with a 6-string bridge (if you can find on that support string spacing that tight).
The pickups would only have 5 sets of pole pieces, but I bet it would sound just fine, regardless.
If the neck is wide enough, it probably wouldn't cost you more than a couple of hundred bucks. Maybe a little more, if you bought the bridge and tuner and had a tech/luthier do the nut, and install the new hardware.
You could do that. But you'd end up with 13.8 mm spacing. I do own a Modulus TBX-6 with 11.5 mm spacing which is Extremely tight (slapping is out of the question) but I'd much prefer something in the 16 mm range like most of my other 6ers. The 6th tuner would have to go somewhere around the eye-poker which is a rather inconvenient spot for it. But my main objections would be the strings not being over the poles and some non-G&L bridge which would spoil the traditional look in my opinion.
Oddly, I did just buy three 5er G&L necks from the G&L store for a song and own a L2500 tribute ripe for experimentation. But somehow the wrong poles and non-G&L bridge ruining the look just kind of puts me off a bit! Current plan for the Tribby and necks is to get out the steam iron and convert one of the necks to fretless for the Tribute.