Apparently not.
Agents have been blocked from making the claim themselves and warned about registering the gateway id themselves and doing it for their clients.
HMRC expect every claim to be made by the taxpayer.
This is, indeed, the case although agents can check to see whether their clients are eligible for the SEISS. Setting up a government gateway account is not kerfuffle free either. There is something from HMRC that says there is an alternative method for the self-employed who can't claim through the internet but they haven't told anybody what that alternate process is!
Neither the SEISS nor the Job Retention Scheme have been properly thought through.
I have a client who ceased employment to start his own business (self-employed) part way through tax year ended 5 April 2019. Because of the timing when he took the decision, his employment earnings (under PAYE) were a few hundred quid more than his self-employed profits. Therefore as his self-employed earnings were not more than 50% of his total income for 2018/19 he has been told he is not eligible for the grant.
So my client gets nothing despite the fact that his income from his sole-trade is his ONLY source of income. He cannot work because of coronavirus and now he gets to live on fresh air. It's a shambles.