Imagine I am working with some old OS installer that thinks that my drive uses first 512 bytes for managing partitions, and it writes to it its data. Can this corrupt my system in such a way that UEFI firmware will not run boot loaders of OS that are UEFI-compatable and have been previously installed to the drive, or will firmware be able to see, once again, previously allocated GPT partitions on reboot?
Overwriting the MBR should never completely brick a regular PC, whether it's "BIOS" or "UEFI"... It may or may not make the OS unbootable, but it will not affect the pre-OS firmware and you'll still be able to boot from some other media and recreate the MBR (or at least reinstall the OS).
That aside, old programs should not cause problems for EFI systems because the 'GPT' partition table (used on most EFI disks) actually starts at the second sector – it reserves the first sector for a so-called "protective MBR", which is there specifically to remain compatible with BIOS systems and MBR tools.
Traditional MBR serves two purposes: the first 440 bytes are used for the BIOS-style "stage 1" bootloader; the remaining 72 bytes are used for the partition table.
The "protective MBR" on GPT-partitioned disks still retains the same format, except it always holds one very large "protective" partition. Your old MBR partitioning tools won't think the disk is empty; they will actually see one giant partition that covers the entire disk, and no decent tool will go as far as automatically deleting actual partitions even if they're of unknown type.
(However, if you do delete it and create new partitions, this can confuse the firmware because now the disk has two partition tables holding different information.)
Meanwhile, tools which overwrite the bootloader area will have no effect on EFI boot process, because the EFI firmware directly looks for a specific file in a specific partition – it doesn't use the old bootcode location for anything.