Maybe, but probably not; what you're describing is obviously a derivative work, multiple iterations deep--one for the photo of the sculpture, another for the 3D rendering & catapult design revision, possibly even another for the further processing & printing. I'd be much more concerned about using letters from real companies' logos.
If it turns out that Claes Oldenburg and/or the Walker have a legitimate claim on designs based on the sculpture (such as your "Siege Engine & Cherry"), and decide to make an issue out of its use because we're making obscene fortunes off our T-shirt sales (hah!) then we can probably afford to stop using it and come up with another design. Mpls. skyline assembled out of capacitors, transformers, tools, scopes, etc... we won't have any shortage of possible ideas.