Sequence on a char(6) column

Is it possible to create a sequence like 'ABC012','ABC013', etc. in a
char(6) column?

I'm toying with some ideas...

I actually need to generate random 6 digit membership number in the format
'CCCNNN' (where C is a character A-Z and D a digit 0-9). The idea is to
have a number like a South African motorvehicle registration (also used
elsewhere) in the format XYZ123. For security reasons a member number like
this should be random and I'm wondering if anybody has done something like
this in a pl/pgsql. I guess a procedure like this could be used as a
default for a column, and making the column unique could prevent duplicates,
although this should ideally be done as part of the generating procedure,
not afterwards.

A sample, or pointers would all be appreciated.

Thanks

Roland