A reply in the support list suggested a not so elegant solution using
a helping table with one row, and after reading some FB1.5
sql.extension documentation:
Before I do anything I should select * from helpertable WITH LOCK
and if this works, it means only I have lock access to that row until
the current transaction ends.

And this I can do in a SP
And this won't cause conflicts with any transactions, that only want
to read from my reservation table and not update it through the SP

And if I want to add some granularity I can dynamicly fill this helper
table with dates, so that the SP will only lock the row(s) that has
the date(s) of the reservation it wants to insert

John