we can't OC in MD, VA has no requirement.
Not entirely true...
You
can OC on your own private property in MD without any permit. I do it all the time, because we own a Condo in MD, and whenever I go up to visit my daughters and stay in the Condo, I carry (sometimes OC, someties CC) the entire time I'm in the house...
You
can legally OC in public in almost ALL the counties and cities of MD (with the exception of the 4 "urban" counties) if you have a Maryland Permit to Carry A Handgun. The MD Statutes that define the limitations of a MD PCH do NOT specify or require that your handgun be carried concealed. However the 4 "urban" counties, and most of their included municipalities have codes that prohibit "public display of a firearm" even if you have a permit...
However, MD does not recognize ANY of the permits issued by ANY other state or territory of the US, and in some municipalities, they don't even recognize LEOSA or FOPA as a matter of practice. So seeing as you are a PA resident, there is simply NO legal way for you to carry in MD anyway, unless you are a cop (and even then, in some counties you will get hassled), or you are a MAJOR political contributor to Gov. O'Malley and can get him to "grease the skids" for you with MSP.
However, let's say you DO have a MD PCH, and you are in a county or city that does NOT prohibit "public display of a firearm". MD statute DOES define holster requirements for transport, if it's not concealed. If you are transporting or carrying a firear in a holster in MD, under MD Statute, it MUST be in a "fully enclosed holster". A Serpa doesn't meet that requirement. In fact, about the ONLY holster that DOES fit the MD requirement is an old WWII military-style flap holster.
The best way to avoid problems involving carry or transport in MD is simply to AVOID MARYLAND if you can. Spend your money and time elsewhere--there are plenty of states in the region (most of them, in fact) that RESPECT and HONOR your RKBA...
And to address the OP, I have both a NC CHP and a PA LTCF, and neither of these states have ANY statutory requirements for holsters (neither do WV, VA, OH, or any of the other OC-friendly states that I know of). I have OC'd a Glock and a 1911 in Serpa holsters in PA, and it has never been an issue...
Carry on, and be proud that you live in a state like PA where (for the most part--except in Philly) they respect and honor your RKBA.