Within a day or two of sending out feeler emails, we had assembled a core group of [blind] players who were willing to assist in any way they could. They provided feedback on our proposed usability retrofit, helped with bug testing, and got the word out to their friends that the game was coming
Diana Hughes of Psychic Bunny, via Gamasutra
Guidelines provide a very useful base checklist but are a one size fits all. Playtesting with disabled gamers is always enlightening, and gives insights specific to your individual game.
Testing as early as possible is critical, as this allows you to put any findings into action for minimal cost.
If you are in a large studio who use recruitment profiles for recruitment, add some disability related criteria to the recruitment profile – it shouldn’t cost much, if anything, to do so. If in a smaller studio, good sources of participants are local gaming groups, local and online disability related communities outside of gaming, and large local communities with a central point of contact for disabled members, such as colleges.