Recently was at a Keilim Mikva which had a sign "No Tevila allowed for outside utensils. Only utensils purchased at this store may be Toiveled here". I was wondering if one goes ahead and does Tevilas Keilim in such a Mikva is the Tevila good? Yes it is wrong to do so, as the owner obviously doesn't want one to use the Mikva unless they purchased an item from him, however at the end of the day does the Tevila work in such a case? (Sources please)
Therefore an immersion as you describe is valid. This is true even though doing it against the will of the owner is forbidden because they can decide how people should use their private mikve.
I checked the above with R Binyamin Tabady who concurs.