I'm not sure I understood your question. But if you don't need or use 
the keeplocal parameter \L, you can just leave it out. If you need it 
though, but also with 0/1 values, then the customsql driver won't you 
help you either.

I just looked at that and it says the following:



This driver allows for custom SQL queries into which variables will be 
substituted. The current FULL username will be provided as \U, target 
address will be provided as \T, the given password will be provided as 
\P, and the option to keep a local copy of forwarded addresses will be 
a "Y" or "N" value provided as \L. If the query to check the 
forwarding status returns no rows, it will be assumed that forwarding 
is not enabled.



I guess a SQL-based substitution would be necessary based on the value 
of \L? But I'm not sure if all SQL solutions provide this.

Why don't you use the customsql driver that offers such flexibility?

I currently use the default setup for vexim which uses a database that 
contains numeric values to determine if a user's mail should be 
forwarded. While yes, vexim does have its own interface for changing a 
user's info, it would be appreciated if the forwards system includes 
the ability to specify what value to put in the database for Yes and 
No within the config file such that it can support 1/0, Y/N, etc.