Hi ExDeeps
In an "it's an idea" sort of way I would say that you are pondering two substantially different forms of part time work:
1. 2-3 days a week
2. Seasonal (or intermittent) work
I think these possibly need somwhat different approaches.
In theory there should be employers who are embracing flexible working and so should look at the CV wanting to effectively job share in a sympathetic way OR they may simply advertise a role as being part time.
To be honest, usually the part time jobs are rolled into full time jobs by adding on environment, quality etc etc. etc.
Though one of my contacts who would not be choosing to go part time did recently ask for my views on a part time role that had been advertised.
In terms of the seasonal or intermittent work, I guess that manu would fall into either the client who probably has enough diversity of needs to make the job full time OR the e.g. farmer who is probably less likely to be looking for in-house staff.
Except for e.g. fairgrounds, where the better end of the sector might be interested in seasonal support.
However, there are regularly contractors (particularly in construction) looking for site [reference removed] for specific projects - which might last say 3 months, but possibly much longer. Might need to travel and might be expected to work long hours for the "season" you are working.
So, if say a site operates from 8am to 6pm Monday to Saturday, your nominal 40 hours could be expected to be much longer. "The site is working so we need a Safety bod whenever the site is open." You might think that being away from home is a chance to explore somewhere new but your employer might have other ideas!!
In terms of what you should do, probably all of the options you have mentioned. By all means talk to the agencies, but be prepared for them to have little competence when it comes to placing H&S people.
....and if you do your networking there might be someone looking for somebody to lead on a short term project. Whatever bright idea someone has that the in-house team has no time to progress.
Good luck, Peter