[TL;DR] vertical rastersplit with different graphic modes is possible with some quirks, in practice one would rather write characters onto a full hires image.
I did some experiments, and found out that it is possible for the VIC chip to switch mode within a rasterline. However a practical application is hindered by the following issues:
- Every 8th rasterline, the VIC chip is stopping the CPU for about 40 cycles. This is the so-called bad line. The CPU is stunned while the middle part of the screen is drawn, which makes it impossible to switch modes in the middle of a badline. This page has a very good depiction of the phenomen.
- After switching between modes it seems that the transition is done in parts. First the color mode is changed to hires while the actual hires graphics appear around two chars later. The following image shows an example where the left side of the screen is in char mode while the right side is in hires mode. The last two characters of "COMMODORE" are already colored with hires function while shown as characters:
That being said, when one can live with every 8th line on the left side being hires (could be colored in background color so it would be not visible) and when the two character wide transition border is concealed, a vertical split would be feasible.
See my question How to exactly sync to the raster line for a vertical raster split? for a short code example.
On a second thought, if the overall mode is hires it is possible to represent any text or blockgraphic from textmode, only printing a char requires 8 bytes to be poked instead of one. However, this is still much less effort than the raster split every line.