Publish without validation score? My mentor wants me to write and submit an academic paper reporting a predictive model, but without any validation score.

Everything I have read in textbooks or the Internet says that this is wrong, but is there any case where only reporting a train score makes sense?

Background

The model was fit "by hand" by someone in our team, using a visual inspection of features extracted from our entire dataset. It is a linear model based on hand-crafted features extracted from some very nonlinear and high-dimensional data. The linear model is based on less than fifty features, but those features were extracted from thousands. We do not have any data left to use as validation.
You are right: decent journals/conferences are unlikely to accept a paper without a proper evaluation. Moreover the model is handcrafted, which probably means that it's hardly reproducible, right? And also that you can't do cross-validation I guess?

I think one would need pretty strong arguments to justify a contribution which has no scientific validation and cannot be reproduced. The only ways I can think of would be:


if the method solves a problem never solved before (or has specific characteristics which make it likely to solve such problem in the future)
if some sort of qualitative analysis demonstrates that the method is much better than state of the art approaches