No, unfortunately not.  Alumail can be forged, but not simply by infusing the armor with [i]caith[/i].  It will take a lot of time for a master craftsman, using master equipment unavailable on an airship.  I will leave it up to you to decide if your character is knowledgeable about this (I'd assume he is), but if you guys want the alumail you will have to defeat the Arbiters while they wear it.  Or if you want to defeat them more quickly you can render their armor useless and attack them.  Or you can use the [i]caith[/i] from one set of armor to help win the fight while keeping one set intact, etc.

EDIT: Also, I would say Lasrach can store alumen in his hammer but it would dissipate fairly quickly, say in minutes.  Unless he uses a hammer that is an artifice designed to be a [i]caith[/i]-well (which would be awesome!)

Double Edit: I do however love the thought you're putting into the encounter!  Makes me feel like a real GM :)  Any other questions keep asking away!