macOS WebObjects - ANY way to use Objective-C?

There is no way that I know of (and it was a big deal when Apple announced that they were dropping Objective-C).

And as you pointed out, there are the earlier versions where it did work. If you wanted to see what it was like (and you have an older Mac lying around) you could buy a copy of Mac OS X Server 1.x (Rhapsody 5.3-5.6). These all came with WebObjects 4.0.1 (both development and deployment tools), so you could give it a spin to see if it is what you were looking for before hunting down a copy of WebObjects 4.5.1 (the last Objective-C version as I recall).

From what I can tell, had Apple not originally thought about releasing Yellow Box for Windows, they most likely would have dropped Objective-C even sooner (like with the release of version 4), but I think they kept it because a number of developers wrote applications for Yellow Box for Windows that would still run on Windows with WebObjects 4.x installed.

During that period Apple was trying to show that Java was as good a choice for regular applications as Objective-C... which is why Apple's TextEdit in Rhapsody is a Java app. Well, Java is not as good as Objective-C. Which is why I use a custom Objective-C version of TextEdit that some developers put together in Rhapsody rather than Apple's Java version (which is really slow).

One place to look for a possible solution to Objective-C in WebObjects 5.x would be over at Stepwise. If you are interested in a historical look at the reaction to some of Apple's roadmap announcements on these subjects, I would suggest reading Head for the Ramparts.