There's a lot of archaeological evidence of eating oysters in abbeys in Flanders and the surrounding, but most of it should date from post-medieval period. Are there archaeological or historical data upon oysters for the medieval times?
Heidemarie Hüster Plogmann, Schlämmfunde aus dem Kloster St. Johann in Müstair. In: H.R. Sennhauser (Hrsg.), Müstair, Kloster St. Johann. 4 Naturwissenschaftliche und technische Beiträge, 2007, Zürich, 227·247.
You might want to check these publications (just a quick look into my archives, there are probably more):
Ayres, K, Ingrem, C, Light, J, Locker, A, Mulville, J and Serjeantson, D, 2003 Mammal, Bird and Fish Remains and Oysters, in A, Hardy, A, Dodd and G, D, Keevill, Aelfrics Abbey: Excavations at Eynsham Abbey, Oxfordshire, 1989-92, Oxford Archaeology: Thames Valley Landscapes Volume 16, 341-432
Ingrem, C. (2003). Chapter 10. Mammal, bird and fish remains and oysters. Phase 4a.The post-dissolution occupation. Aelfric's Abbey: Excavations at Eynsham Abbey, Oxfordshire,1989-1992. Thames Valley Landscape Volume 16. A. Hardy, A. Dodd and G. Keevill. Oxford, Oxford University School of Archaeology / Oxford Archaeology.
Kunst, Günther Karl & Galik, Alfred (2000): Essen und Fasten in mittelalterlichen Klöstern aus archäozoologischer Sicht. in: Adler, Horst (ed.): Fundort Kloster – Archäologie im Klösterreich, Fundberichte aus Österreich Materialhefte A 8, 249-258, Wien
Morris, G. (1990). “Animal bone and shell”, in Excavations at Chester. The leser medieval religious hoses. Sites investigated 1964-1983. S.W. Ward. Chester City Council. ISBN: 1-872587-00-3. pp.178-190
Murray, E., 'The marine molluscs' in E. Bourke, A. Hayden & A. Lynch (eds) Skellig Michael, Co. Kerry: the monastery and South Peak. (Dublin, 2011), pp 450-51.
Sorry for the delay - i wasn't awaiting any further answers. Thanks for the publication list. In the meantime an article upon the oysters at the Abbey of The Dunes in Flanders has been published: literature (bestiaria) was available in the abbey, but oysters themselves (and archival evidence for them) seem to date from the 16th century onwards.