Here's an idea - how about adding vet reward choices to the store, while revising the list of the Vet Reward items?
1. Use sovereigns to buy extra vet reward picks at your account's age level, for, say, the account's age in months x 100 sovereigns (with those old enough to get 2 a year, getting two for that price).
2a. Move all the clothing items to the store, and change them to cost 100-300 sovereigns each. Tweak the item IDs for the store ones, so that that the system can distinguish between them.
2b. Make reward dye tubs available in both store and vet rewards. Make all the first year dye tubs count as 1 pick for the set, with item IDs that are different from the originals. Tubs/tub sets would cost 500 sovereigns x the item's vet reward age in years.
3. Set up turn-in crates that allow the conversion of the pre-change clothing items and first year tubs into either other like items at a higher level, or into sovereigns (at 20% of the store value of the item). So, a sufficiently old enough account could trade in a copper cloak for a pink dress, or for 20 sovereigns.
4. Add "vet reward sets" to the Store, tied to the character's account age. For an outrageous amount of sovereigns (probably a 5-digit amount), if the account is old enough, you can buy a set of one of EVERY non-clothing reward for the account ages. For example, an 80-month account could buy a 12-month, 24-month, 36-month, 48-month, 60-month or 72-month set. Each set are items specific to that year of age (IE 36-month does not include the stuff from 24 or 12 months), and may be labeled as "replicas". No substitutions, and if an item is customizable, you only get one of them (banners, for example). The irony is that the first few levels of these will probably be the most expensive, due to the sheer number of 1-year and 2-year items. Past a certain point, the price would be driven by the equivalent costs for X number of picks, from #1 above.
5. Everything else - see #1, buying an additional vet reward slot (or having someone else get one for you).