There is still a requirement to check space on the C: drive, as this is where the temp area is normally hosted, and where the MSI will be uncompressed (assuming you have internal CAB files) prior to installation. In addition, the C drive is typically the system drive, and the MSI will be cached there, and all registry entries relating to the install will also need to be recorded on the C drive.
If there are any DLLs to be installed on the system partition, space will be required for this also.
Consequently, removing the space check for the C drive would be risky, to say the least, and I don't think you could achieve this without disabling space checking on all volumes.