Custom Data Type Question

I'm trying to create a custom data type similar to an enumeration type. 
However, I'd like the mapping of the int<->string to be dynamic instead 
of hard coded. I'd like to have a table that contains this mapping that 
can be appended to. Creating this type is not very difficult. However, 
for performance reasons, I'd like to cache the mapping so that the table 
is only queried once every connection unless it changes. I'm thinking a 
combination of a flag that can be triggered on insert and a transaction 
id could be used to decide if the table needs to be reloaded. 
Unfortunately, I'm not exactly sure how to get started on this, any ideas?

Thanks,
Greg