It does seem a little funny for some reason the way it works for bass. It seems much easier to get over 100k without leveling up all the phrases of a song on bass. But, MiniCoop got a bass arrangement mastered last night, so it is possible.
It seems like I have gotten all the phrases of a few songs leveled up (based on the boxes at the top of the scoring screen on songs) without even trying hard - but then I can't get over 100k. Just a slightly different deal, I guess, possibly based on the fact that you just won't have as many notes in most bass arrangements as we're used to seeing in guitar parts - thus each note will be worth more points. Miss a couple of notes and you tend to see a bigger difference in scores at the end of a song than what you'd see on the guitar arrangement of the same song. Just a guess based on the previous analysis of how RS scoring works.
I haven't done enough on the bass yet to worry about mastering songs, but I was happy to see that there's still a master mode for bass. I'll work on that later when I've got a little more practice under my belt. That'll have to wait for the huge blisters to go down on my 1st and 3d fingers!