I read that as "(You are considered proficient with the improvised weapon and are considered to have Weapon Focus with improvised weapons) for the purpose of meeting the prerequisites of feats that specifically select a weapon," not (You are considered proficient with the improvised weapon) and are considered to have Weapon Focus with improvised weapons for the purpose of meeting the prerequisites of feats that specifically select a weapon, such as Weapon Specialization.

The latter reading might be possible, but even then I don't think it works. Because you're not proficient until you are using it, and you can't create it unless you're proficient in it.