We've tried extracellular BioID several times in our lab, but we haven't had much success. There seems to be something inhibitiory about the EC environment, even without serum or in HBSS. If you'd really like to try something like BioID extracellularly, you could try TurboID, which could have some success since it's much more robust. Otherwise you could try an approach like EMARS or SPPLAT.

Thank you for your response and sorry for this very late feedback.

For those who may interested, fusion of your P.O.I with HRP appears to be usable for such experiments as it work at pH compatible with extracellular proximity labelling with biotin.

You can an exemple there: https://linkinghub.elsevier.com/retrieve/pii/S0092867416309916