Lots of good posts here.
To answer the OP, I would say that yes, you could selectively choose to lower your AC to receive an incoming bomb. Maybe instead of standing perfectly still to receive the bomb on the chin, couldn't it just as easily be explained as intentionally throwing a limb in the direction of the bomb at the last second?
In my game I would rule it as AC 10+environmental mods, i.e. cover etc.
I feel the strongest argument in support of this would be that people are not expected to dodge touch healing spells, why would a ranged heal be any different?
Especially since the Alchemist could just even have a code word with his friend, if more verisimilitude was required.
Also, earlier it was mentioned that various combat maneuvers could be used against your friends to gain benefits.
I believe that these are all strictly the purview of DM judgement.
I think all combat feat benefits can be turned off and on at will, but the nature of the rule must be preserved. To give an example, I'll use bullrush.
Normal bullrushing, an opponent does get an AOO, but if your friend is bullrushing you with your consent, then no AOO has to take place.
Now, the reason why a normal or improved Bullrush's movement does't provoke AOO is purely a game mechanic to make Greater Bullrush more rewarding and allow the feat chain to become more viable offensively. (Really, how much different would a rush be that it would provoke vs not provoke? Maybe this is a failure of my imagination.)
So in keeping with that spirit, I'd rule that yes if you are rushing your ally along, both of you would provoke AOO from your enemies. Improved Bullrush or Greater.
I cannot support clear reasons why in the rules this would be allowed, but it would in my mind it somehow preserves the balance.
That's why Pathfinder does not need exhaustive FAQs, or piles and piles of in play examples, balance is preserved by DM Fiat with a healthy dose of common sense.
The rule books get to be the reasonable size they are because we have a human brain to fill in the gaps.
Plus, I'd find it extremely entertaining to imagine just how these co-operative maneuvers would play out. You can flavor it in so many ways! The ally could be shoved out of the way, barely escaping harm, to a roaring warrior using the momentum of his ally's strong push to launch a devastating charge/pouncing attack. (not referencing the action or special ability here, just description.)
Thanks for reading.