Strange, the algorithm is very trivial:
It basically takes 4 pixels and makes 5. Repeating this 64 times you get 256->320 enlargement.
pixels before:
| 1 | 2 | 3 | 4 |
pixels after:
| 1 | (3/4 *
1) + (1/4 *
2)
| (1/2 *
2) + (1/2 *
3)
| (1/4 *
3) + (3/4 *
4)
| 4 |
Have no idea what it could be called, probably just trivial pixel averaging, I thought of it myself

The 1/2, 1/4 and 3/4 multiplications can be achieved without float arithmetic by simply shifting, so they are very cheap.
This can be easily applied for the SNES emus, but it would be slower there, because it would need to be done after the whole image is rendered, not while it is being rendered (what I do in PD).
Edit: oops, I messed the formula, the one actually used is which ledro wrote below