The rims don't limit the width of tires that you can use. I used to race XC on 45c Panaracer Smokes mounted on MA40 rims with 13mm inside width.

A better question might be "how (actually) wide of a 27" tire can you find" or "how wide of a tire will fit the Panasonic frame and fork".

Note that the stated size is only loosely related to the actual size of the tire, and that the tire's width is affected by the rim's width.
I've seen 27x1-1/4" tires measuring as small as 26mm wide and as wide as 33mm wide, depending on the particular tire and rim.
I've used 27x1-3/8" tires that measured only 29mm wide, and I've never seen one that measured wider than 33mm wide.

The 27x1-1/4" Pasela tires seem to run as wide as any, actually true to inflated size or perhaps a mm or two larger, depending on the rim width.