Support for UF2 file format

[oceanofthelost]

I am working on adding UF2 file format and am stuck in a couple of areas:

1. UF2 supports upto 476 data bytes, but from reading srecord code, looks like internally we hard code the maximum as 256?
2. Total number of blocks is required to know upfront. Output looks to translate data on a line by line basis (internally data stored as srecord format, 1 line is one record), with no knowledge of how many lines may be written.

I can work around 1 by setting the max supported output line to 126, which for now is okay as this will still meet s the spec.

2 is not something I think I can work around currently, I do not see a way to inform the output on the number of record objects that will be translated.

Any recommendations or is this a file format something unsupportable by srecord at this time?