Orange-Green specularity?

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Is it wrong to use greyscale specular maps if they work how you want them to? Or is it more of a "standard" to do the RG channels?

It probably works, but it will give you the same values for gloss as for specular. A greyscale picture that has been saved as RGB has the exact same map in every channel. HPL ignores the blue channel, so it just gets all the information it needs from the red and green channel.
I'm not sure what happens if you save it as an 8bit (only one channel) however, since in that case HPL is missing a channel that should be there. o.O

So, as long as your specular and gloss map are supposed to have the same values you can do it like that. There are cases however, where you want to have different values... wood for example has low spec and low gloss > would work fine with your method. Smooth plastic has high spec and high gloss > would work fine too. Brushed metal on the other hand has high spec and low gloss - here you'd need different maps for the best result.