Dogscoff,
Assuming that someone 'trusts you' because they have a treaty with you is a bit naive.

Let's just let the target's counter-intel level determine the chance of success.
Other than the modifiers I have recommended before, of the race on the planet, and their current anger level, having an effect.
It would be nice if the planetary 'covert recon' report would tell you the race of the inhabitants and the 'mood' (anger level) on the planet as well as the cargo and facilities.
I would also support the idea of being able to plant 'moles' in other empires. You could spend points on the 'mole' project and if it is successful you'd have those points stored for possible use in a future intel project in that empire.