MDCT is an overlapping transform.
copy /b f1225.mp3 + /b f1225.mp3 longbeep.mp3
With a slightly more complex procedure (than the one that started this thread), [...]
Gapless decoding, on the other hand, is kind of impossible if your decoder can't skip samples. It will need to chop off the specified encoder delay, and compensate for its own delay, and then truncate the file to account for the remainder of the padding. If it cannot do this, then it will at least be able to start the stream over, but it won't be gapless.