Is it possible to mirror and weld a small portion of a mesh? I tried to mask a section out but it mirrors that too.
I don’t think there is a tool for doing so. But you can get identical results with:
Mask the part you want to protect, use split masked. Now use mirror and weld and merge the two parts again. You could use weld verts to restore the full mesh integrity and then fill holes if you want to fill the holes that probably will be created by the irregular mirror operation. Most of this can be done in a single macro leaving you with the only task of masking and using the macro. I understand this could be very useful for zmodeler when working in partial symmetry.
Notice that if the differences of both sides are of shape and not topology (same verts) and the object was initially symmetric then you can use deformation-smart resym. This accepts masks, in fact if you don’t use them it simply will do an average of both sides. You need to mask here the parts you don’t want to change and the unmasked will copy the masked ones.