Right now, it is not possible to equip harvesting tools during combat. I can think of various reasons why the do not want players able to equip/change armor or weapons during combat, but I can’t think any reason for harvesting tools.
This becomes fairly annoying when you are running about harvesting, have aggro’d something, and your harvesting tool runs out (more likely when at rich ore nodes).
I’ve also had cases where I’m ‘in combat’, but am not being attacked by anything (because the creature can not get to me or some other reason). I then have to hunt down that creature, kill it, so I can equip the item, go back to the node, and finish up harvesting.