UPC standards a prerequisite?

I don't see why you couldn't. The wording says to use an "aggregate" of 20% less water. I don't see anything saying that one fixture can't use more water.

I think it will be challenging getting 20% water savings with a lavatory faucet using more than 0.5 gpm. I think it's your option to try.

It's also the IPC (International Plumbing Code), the EPA and the American Society of Mechanics standards and many local jurisdictions have adopted these codes and standards as their own code. Check your local code. You might be required to use a 0.5gpm faucet anyway.

Other than that I agree with Bill above that one fixture could use more than the baseline as the aggregate of all fixtures required to be included is the key for the LEED calcualtion, but I think it would also trigger a red flag with a reviewer. In any case you are measured against that baseline, meaning if you use more it only hurts you and the other fixtures have to make up for it.

I just got a review back for two sister buildings. Both are using some fixtures, which are above baseline, but the aggregate consumption shows a reduction of more than 20%. The one building was approved in the review, the other wasn't because: "However, the flow rates for the installed lavatory (0.66 gpm) and managing directors shower (4.76 gpm) exceed the maximum flow rates recommended for the fixtures by the EPAct of 1992, 0.5 gpm and 2.5 gpm respectively. TECHNICAL ADVICE:
Please provide a revised form demonstrating that the installed fixtures, including the lavatory and managing directors shower, at least meet the maximum
flow rates recommended for the fixtures by the EPAct of 1992."
I very much agree with the others above and don't see any reasoning for this denial within the LEED requirements. Also these projects are both in Germany, where we don't have a requirement for water consumption or maximum flow of fixtures.

I would appreciate any comment especially from the USGC or GBCI to clarify this.

The BD+C rating system indicates that the project must “employ strategies that in aggregate use less water than the water use baseline calculated for the building.” I see no problem in adjusting certain design case flow/flush rates to be higher than their respective baseline numbers, as long as the baseline case flow/flush rates remain at EPAct levels and the aggregate water use reduction is at least 20 percent. I recommend that you contact the GBCI regarding the discrepancy between your “sister” project reviews.