CodesInChaos already commented in the first question linked to. It seems logical that having the most significant bit set to zero during generation of the random value $z$ to not impact security all that much.
Still, the use of random number generators for large amounts of data can be a draw back of the mode of operation, especially when they are required to generate 4096 bits of randomness for a single RSA operation. This could be an issue if said random number generator is slow or if insufficient entropy is available to the random number generator.
My related questions are:
Can we reduce the amount of input keying material to the KDF further without reducing the security of the RSA operation? If yes, by how much?
If yes, is there a good distribution of random bits required (half to the left, half to the right?)
If no, could we use a one-way primitive such as a XOF or indeed MGF-1 after retrieving the required number of bits for secure key derivation, and feed that to RSA-KEM?
Or should I just run away and use RSA OAEP instead if I run into situations where a fast DRBG is not available?