Re: Okay...so I'm hooked on the Youtube Smooth Jazz backing tracks....
Subs that come to mind:
As noted, B minor pent or blues scale.
Bm7 arpeggio
Maj. 7 arp. up a 3rd (Gmaj 7)
Maj. 7 arp down a 2nd (Dmaj7)
Mix up the above with the obvious choices (Em7 arp., E Dorian, E phrygian, E Aeolian).
Scales are arbitrary groupings of chord tones and different extensions, but what matters is the chord tones. So as long as you emphasize correctly, or use a good sounding arp sub (see above), it will work. it is boring to stay in one scale or arp. for very long; and obnoxious to stay outside too long.
You'd think Bm blues would sound bad over Em7 since the b5 of that BmBlues scale is a flat 9 of the Em, a half step off the root, but it doesn't if you use it as a passing tone between the root and 9th and stay in Bm or Bm7 arp long enough to establish it. IT sounds really cool actually, and briefly implies both E phrygian and E Dorian/Aeolian.
And you don't even have to stick to that. You can mix in the 2nd of B into that blues scale, so you can use B C# D E F F# A as your scale, or just add the b5 of the B (F) to the B Aeolian scale (Avoid B Dorian as the maj 13 becomes a maj 3 over the Em, which sounds bad). All that becomes D Major with a passing tone between 2 and 3, which is the basis for E Dorian anyway. So keeping to just a pentatonic or blues scale is, again, an arbitrary grouping of chord tones and extensions over the Em.
Arp subs are like scales, they are really just groupings of chord tones and upper extensions that you can limit yourself to in order to get a certain effect. The more upper extensions in the arp sub, the more it "Floats". Over Em7, Dmaj 7 (which has the 9th, 11th, 13th, and 7th of Em7 and no triad tones) floats more than Gmaj 7 (3rd, 5th, 7th, 9th of Em7).
Personally I think in scales only when I'm doing a very fast run, and even then I'll often use an arp. sub mixed in or instead of a scale. Think of the chord tones, then think about how each of the other tones in a chromatic scale sound around them and go from there.
The only note I'd stay away from over a minor or m7 chord is the maj 3rd, but I've even seen some folks use that to good effect.