You're right about reserving a value generator. It can safely be done in the same connection than the Insert.
If my memory is still good, db2 do this and have by default pages of 4K. MySQL have a real row lock (others I don't know)... Considering the small size of our sequence table the chance to face this situation is high.
eg. in my app, I have 22 sequences for a total of 194 characters (the entire table is stored in one single page)