Apparently London Bridge had a chapel in it, which means that in an Ars Magica sense, it had a Divine Aura. It also had a band of men who served the chapel and kept order on the bridge, and shops and residences. I think a covenant built on the bridge would be an excellent setting for a game. There’d always be something going on.http://upload.wikimedia.org/wikipedia/commons/thumb/6/68/London-bridge-1682.jpg/300px-London-bridge-1682.jpg
Games From Folktales 
I had the same thought about the bridge in Florence, the Ponte Vecchio, but that the river might provide a lacuna, allowing them to have a Magic aura in the middle of the city.
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I’ve always liked the concept of “Old” London Bridge with all its houses and community. Reminds me a lot of the Bridge community in William Gibson’s Virtual Light / Idoru / All Tomorrow’s Parties “Bridge” Trilogy. An excellent inspiration for a covenant!
Actually quite a lot of medieval bridges seemed to have chapels – the Pons St Benezet in Avignon has a chapel for example. There was usually a small brotherhood assigned to an individual bridge, responsible for its upkeep and collecting tolls etc – this brotherhood would make a good Agency or even covenfolk for Hermetic magi.
The Divine aura generated by a chapel does create potential problems for a covenant (particularly in a major city), but not ones that can’t be overcome given the existing (and forthcoming…) examples of covenants in cities. A major bridge not in a city or town (but with an associated village and inn etc) would be an excellent covenant or perhaps Mercer House option.
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It is indeed quite common for medieval and renaissance urban bridges to haves chapels (to protect the bridge against the devil that would be quite strong in such an area) and shops (because people are passing by). It was also the case in Paris until the XIXth century for instance. A “simple” passing bridge in a city would be such a loss of space!
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