Sounds like brick acid is a sensible solution in this case.
IMO brick acid is hardly ever a sensible solution indoors :-) Horrible stuff to use, and hazardous to store the remaining 4.9 litres. Given that powdered filler is cellulose based (and thanks for looking that up), I'd guess any acid would dissolve it. I can't imagine it would cope with permanent immersion in water though, so whatever the builder put down there was more likely to have been an acrylic cartridge type filler.
If it's a choice between junking the pan and giving it a go then I think we're big enough here to take sensible precautions (picture killing room setup from Dexter ;-).
As it says the cellulose is a reinforcer I reckon it's safe to treat it as being mainly 'white plaster' ie gypsum and attack it with HCl. Further googling suggests that hot dilute HCl is recommended to dissolve gypsum, perhaps with repeated apps to wear it down.
On the acrylic debate, I think it is more likely to be an overmixed quantity of powder rather than cartridge waste as there'd be no reason to over dispense the cart stuff in the first place. Andy will know though so this might help.
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