How much weight can an old work box hold?
From what I could find, it doesn't look like these types of boxes are listed for any type of fixture support.
When looking at a box that attaches to a stud/joist using nails (B520P), it states.

Suitable for fixture support up to 50 lbs.



With the the old work version(B618R), it states.

Not listed for fixture support in ceilings.



A box like this (BH614R), states.

Listed for wall fixture support up to 10 lbs.


The last page of this catalog shows some round boxes, and the weight they are rated to hold.
Based on this information, I would say the box you want to use will support less than 10 lbs.
The best options in your situation, would be to move the fixture location to allow you to fasten the box to a structural member,  or add a structural member to mount the box to.

Update:
I contacted a company that manufactures these, and here is their response.

[Link to product omitted]
As shown on the link above this item is not listed for fixture support. We do have several other boxes that are rated for fixture support.
[Link to other product omitted]

The link for boxes rated for fixture support, pointed to boxes that attached to structural members.