Looks like most of the emergency "camping" toilets have weight limits of 250lbs. I know that lining an existing toilet with thick bags would work as well, but that doesn't provide much room for expansion.

Are there emergency toilets that will support over 250lbs?

I am not sure what an emergency toilet would be, or for that matter, its close relatives, the survival toilet, or the tactical toilet.

When there is no toilet - prepare the receptacle or space (the so called dumping ground), squat, and function. Fill in, clean up, and leave no trace! Be sure and wash your hands....

The emergency toilets that I have seen consist of a folding frame similar to a folding chair, but with a toilet seat under which is placed either a strong plastic bag, or a bucket.

Much easier to use than squatting especially for the less fit or those unused to having to improvise.

These light weight folding articles are indeed unsuitable for those over about 250 pounds or 110 kilos. Different makes vary a bit.

I have prepared as follows at little expense and with minimal space taken up.
I purchased a piece of heavy duty plywood about 30 inches by 45 inches, in the middle of this I cut a toilet sized hole.
Over this hole I fixed a standard plastic toilet seat complete with hinged lid.
The two longer sides of the plywood are reinforced with square timber screwed to the under sides.
When not in use this contrivance may stored on edge behind crates of other supplies and thereby take up little space.
When required for use, the board should be supported at each end on strong stacking crates containing doom supplies, choose crates to give a comfortable sitting height.
Under the seat I would place either a bucket containing cat litter or other absorbent material, or a bucket lined with a strong garbage bag. Ensure that only a minimal gap exists between the rim of the bucket and the underside of the plywood.

The plastic seat is readily cleaned if it becomes soiled.
The level surface to left and right of the seat would be handy for placing a toilet roll and a flashlight or lantern.

The supporting crates may still be utilised for storage, I would not much fancy keeping food or drink thus, but toilet paper would be fine.

The cost was about £25, excluding the strong crates that I needed anyway.

Something like this might suffice for use with a standard chemical toilet..

https://www.bigjohnproducts.com/cm.htm

If a regular bucket emergency toilet has limit of 250lbs, just put it inside another bucket for a limit of 500lbs?

http://fivegallonideas.com/cinder-block/ says weight limit is 550 pounds with gasket lid can support and snap lid 308 pounds

This compares to what Luggable Loo says weight limit 250lbs

So just double up the buckets to double the limit an extra bucket is $3

OTOH, $79.95 supports 500lbs PETT camp toilet
$87.10 hold up to 350 lbs Folding-Bedside-Commode

OTOH, for ~$700 a full sized "Porta John" folding portapotty... ought to support 500lbs easy

Then there is always DIY milkcrate or plywood humanure toilet

$3 bucket + $5.74 seat + $1 screws + $1 hinges + $5.97 plywood = total $16.71 in materials

Thanks guys!

I meant to reply this morning when I suddenly remembered there are chairs that go over toilets and saw that Am_Fear_Liath_Mor posted that, but didn't get a chance to confirm it.

I think the best bet for the use-case I have is what Am_Fear_Liath_Mor posted which is similar to the Folding Bedside Commode that EMPnotImplyNuclear posted as well. I believe you can remove the bucket and put it over a normal toilet as well.

In days of old
When knights were bold
And toilets weren't invented
They laid their load
Right in the road
And walked away contented.

To Limerick scatological?
Some would sniff: pathological!
Though it takes small wit
to rhyme of ----,
the subject's ontological.

(I'm working on one with "coprolite" but getting nowhere. I think it needs an archaeologist's touch.)

Tsk, no takers? Dreadful pity.

Back to the OP:

It seems to me that any chair that could normally handle the weight of a person of substance could be easily modified for essential functions, with the receptacle moved to suit the situation.

I recall images of an extended situation in ??Australia?? where the creative use of lawn chairs and pits for personal business was quite inspiring. Can't find a link offhand, but it's out there.

I am sure an archaeologist would really dig your verse...