One useful soft-working developer I have (which doesn't necessarily mean
it is still available) is Tetenal Centrabrom-S liquid. Normal working
dilution is 1+3.
I have used this with various Agfa graphics arts films (they were kind
enough to send some free samples) and also Kentmere Polyester film type
HSP-C(Clear) which is (or was) a rather cheap film. The box of this has
written on it by me: for positives: 15s f11 dev 3 min CentrabromS 1+9
for negs: 10s f11 dev 1m40s Ilford multigrade 1+9
Hasten to add it will only give a neg if you expose from a positive.
I used to make a soft, well-exposed positive, this could then be contact
printed to give a more contrasty neg if required - I think the second
developer was used to give appropriate contrast for platinum, I would try
the first one for negs for gum.
However these might be suitable starting points for experimentation
Peter
petermarshall@cix.compulink.co.uk