A dev will take you up on that, probably!
Devs currently have a different plan
Rather than trying to derive a rule when it's safe to change newgrfs, the plan is to throw away scenarios as we know them.
Scenarios are just savegames, changing that to make newgrfs changeable is a very deep pit filled with red-hot lava. You really don't go there even if life depended on it. So instead, we want to change what "scenario" is. Rather than a world that you can load, we'd like a scenario to be a description of how to assemble the world. You have to provide a bunch of files for this purpose, a heightmap, a road map, a water map, list of industries, list of towns, etc.
When you load such a scenario, the program quickly interprets all those files, and builds the world for you as you described it.
First idea of that plan is at https://wiki.openttd.org/Terkhen/Scenario_format
Of course, the OpenTTD scenario editor will be able to load the above descriptions, allow you to edit it, and save the files again. However, the description is designed to be open, as it uses standard file formats, like PNG. This means you can make a scenario by just providing those files in any way you want. In other words, you can use whatever software to make each of the files, assemble them all into a scenario, then load and play. So if you want to have real-world placements of cities or roads, write or use software that generates the right file data, and it will work.
How well things work if you make changes in the description is one of the things that is not known yet, but much depends on the format of the description. Some form of NewGRF list will be in the description, and it seems feasible to allow editing of that list before the world is constructed. How well that actually works is not yet understood though, but much depends on what you can say in the world assembly instructions.
If it can only say "Place FIRS2 industry #26 at (x,y)", changing FIRS by another version of FIRS or by another Industry GRF would be problematic. On the other hand, if the assembly instructions say "Put an industry that produces COAL at (x,y)", that would be more easy to accommodate.
This holds for a lot of things. Saying "I want a city of 30 houses at (x,y)" is more flexible with changed newgrfs than "city has house 32 at (x1,t1), house 35 at (x2,y2), etc".
If you extend this idea further, and give more global descriptions rather than the exact placement at each and every tile, you could get scenarios that are different each time you play them. For example, if you could say "Make 5 coal industries in this area please", that would be really nice.
Such global instructions could also work at different map sizes, if you allow scaling of positions to map size.