How to get all PDB homologs from Uniprot (mapping + BLAST)? I'd like to create a dataset consisting of all sequences which are either present in the PDB, or whose homolog is present in the PDB. In other words, any sequence in the PDB or any sequence related to it. The similiarity margin is to be very wide, so anything above, for instance, 25% sequence identity is accepted (perhaps there are better criteria, but you get the point). 

No further limitations, so any organism, with or without experimental data, not limited to the manually curated Swissprot set. 

I have tried the SIFTS database, but that contains, as far as I can tell, only mappings for sequence identity >85%. I would like to go far below that.
For the correspondence between PDB and Uniprot entries you can use SIFTS -- a semi-automated mapping between PDB and UniProt maintained by PDBe.

The pipeline that creates the mappings uses BLAST and a few other criteria to decide which UniProt entry should be assigned to each PDB entry. The SIFTS website has all the data in CSV files.