Add custom properties to a column's definition (pg_attribute)

Can anyone suggest a way to associate a set of custom properties (key/value
pairs) with a column's definition (ie pg_attribute)?

(without having to replicate/maintain a table of columns separately from
the system catalog)

For example:
* I have a table with 50 columns
* I'd like to 'flag' 30 of those columns as group - 'A' and the other 20 as
group - 'B'

So I'd like a way to add custom metdata (k/v pairs) to a column's
definition (not the actual rows of data).