For some reason, I feel like Bardic Masterpieces would be better as feats that were available to any character who meets the prerequisites.
Accordingly, the Bard would receive bonus masterpieces. I was thinking one at 2nd level and every three thereafter (5th, 8th, 11th, 14th, 17th, 20th).
Obviously, the cost in rounds of performance for many of the masterpieces would need to change, in order that it might be performed by non-bards. I'm thinking that the entire category of Masterpiece feats may carry some kind of limitation equal to perform ranks.
What are the obvious and subtle consequences of such a change?
I like where you are going.
I would likely leave the masterpieces as written for most characters who meet the prerequisites (which should be mainly set by perform ranks, and maybe CL for some of the more "magical" abilities), but not have them require rounds of BP. For Bards I would give the option to improve the benefits granted by the masterpieces by sacrificing a number of rounds of bardic performance. I would also say that unless he is boosting the Masterpiece using rounds of BP, it should not interrupt his active BP.
However, I would not give these to a bard as bonus feats, I would make them feats more worth taking for a bard. The abilities are flavorful, but when masterpieces give a flavorful effect in trade for a functional effect, it really ceases to be worth it.
One other change I might make is to allow more than one Perform skill to be used for any of these, not just a select few (maybe an option of 2 or 3). This will also make them more accessible.