If you go to the
pixelpost.lrdevplugin folder and open up the file
PixelpostExportServiceProvider.lua you can comment out line number 35:
Code:
-- allowColorSpaces = { 'sRGB' },
If you add the two hyphens (--) and save the changes it should allow you to choose between AdobeRGB & sRGB when you export. Of course, I highly recommend that you stick with sRGB, as that is the current web standard.