The Gatherer mod would be your best bet - you can configure it to display previously found herbs & ore veins and have it display on your main's mini-map.

The only catches are:

1. The herb or ore vein has to be 'discovered' first by either yourself, someone in your party, or someone else in your guild who is also running the gatherer mod.

2. If your main doesn't have the profession, and you're not monitoring your other toon's mini map, you'll need to get within visual range to determine if the herb or ore vein is actually there.

Still, once you had a decent gatherer database populated, this would be pretty close to what you're looking for. Gatherer also populates your main map as well, so allows you to plan your route in advance if you are doing a herb /ore run in a particular zone.

Cheers,
Stealthy