I didn't play the first dotp but from what I've seen so far dotp 2012 reads data from folders, however the priority is set to read from the .wad first, changing the priority would allow data to be read from folders first, but I haven't check this yet. Don't know if Kev or Gibbed have already checked that out.
Gibbed as already pointed out how to bypass the .wad checksum (
http://www.slightlymagic.net/forum/viewtopic.php?f=62&t=4832&start=15#p62950).
Don't forget that the offset he's talking about is a memory offset, that is, the offset after the game has been loaded. If you want the real offset you have to subtract the memory offset to the address in which the game has been loaded. Typically the executables are loaded in 0x00400000, so the real address would be RA = 0x0064B3FF - 0x00400000 => 0x0024B3FF. Please note that I'm not sure that dotp is being loaded in address 0x00400000, so the real address might be wrong, but later on I can confirm you this. However you might give it a try, if you find the byte 0x75 at 0x0024B3FF then just change it to 0xEB.
Hope this helps.
EDIT: By the way, the patching method only works if you have an unpacked version of the executable, that is, you either unpacked steam_api or you downloaded the SKIDROW version. If you have bought the original game and haven't unpacked the executable this won't work. Unfortunately, the process of unpacking is rather complex and hard to follow if you haven't reversed engineered any game before, so if you bought the original game download the SKIDROW crack but remember to remove it when connecting to steam otherwise you will get banned! Also, if you modify the executable in any way and connect to steam you will get banned! The above information is for offline mode only!
EDIT2: Just confirmed, the real address is 00x24A7FF and not 0x0024B3FF. The reason is that I forgot to subtract the code base.