You mean one usable for 5.x and one for 6.x, right? +1, but to make it simple I would just keep 'extra' as-is for 32bit and use maybe just a simple 'extra64' for 64bit...
Also keeping them separate would allow easy setup multiboot 5.x/6.x on same flash, useful for testing at least
Couldn't it be easier to maintain one single .plg file (with proper simple script checks added for unraid version, to install the right packages for it, etc) and just add some xml tag on the plg file specifying compatible unraid versions (if no tag present take it as 5.x compatible only), and when trying to install an incompatible plugin just ignore it...? that would still prevent installing incompatible plg's and make it possible a single .plg file to be maintained, compatible with multiple unraid versions (even thinking on future... yet newer unraid will hopefully eventually come again, based on an yet newer slackware or other distro, etc that would require yet different packages again even if also x64...) I mean something like firefox plugins minver/maxver compatibility check or similar...
Anyway I do also surely agree with some way of .plg separation, even if just file extension based.