If I understand your plan, you want to protect the design with a finish that cures and protects while using a less effective finish on the rest of the surface.

At best, you will have different appearances to that entire surface between the two finishes.

At worst, the possibility of the wood reacting to humidity changes and the two areas moving differently.

If the oil is fully cured, shellac will stick. I won't rule out that is wouldn't crawl within the next 100 years.
Casey

As said, shellac will stick to cured oil, at least a week in ideal conditions.

Shellac will have a high sheen and possibly a film build, depending on application. Oil will have neither.

I didn't think about it, but shellac does have a high moisture excluding efficiency (MEE), linseed oil none, actually hygroscopic (attracting moisture).