The problem with creating a Rod of Heighten Spell is that the feat doesn't have a set cost in spell levels. Empower is +2, Quicken is +4. Heighten is....whatever you want it to be, with a sliding-scale effect based on the cost paid.
A DM might create a Metamagic Rod of Heighten Spell by assigning it a set level bump, then pricing it accordingly. For instance, you might have a Rod of Heighten Spell (+2) or a Rod of Heighten Spell (+4). Then each Rod would have the standard three versions--Lesser, Standard, and Greater.
And then these would all have to be priced.