I am not going to attempt removal of the foam as it entails disassembly of the unit and modifying the air housing. I would fear that if Philips chooses to repair the DS1's by replacing the air housing that an obvious modification to the unit may void a repair since the unit has been internally tampered with. (I hope those that have disassembled the machine had the forethought to wear an anti-static wrist strap or similar if handling or touching the circuit boards.)
I've given thought to an inline anti-bacterial filter, but have seen a few reports that the filter alters the air flow and users have cited the machine is not appearing to function the same as it did without a filter. The DS1 machine appears to have no compensating setting for use of an inline filter which I believe the ResMed may have. The humidity feature with a heated hose would be problematic as well with an inline filter. To filter out particulates an AB filter is overkill.
So I'm playing with an idea of adding a filter of sorts, primarily to capture particulates, by fastening a fine nylon mesh food grade straining cloth (150 micron) over the air port within DreamStation water tank. It easily passes air and will not restrict the air flow or humidity, but will capture fine particulates if any should break free from the foam. At the moment the straining cloth is affixed to the air port with rubber bands. While the cloth and rubber bands are on quite securely I'll probably look to improve upon this in some way or use the straining cloth in another manner to filter the air flow. Yes I've considered the cloth and rubber bands could fall off and looking at ideas to avoid a 'problem' .