I found a very compact solution that requires only two memory writes to enable the bar:
It uses a combination of Extended Color Mode (ECM) with Bitmap Mode (BMM), which the VIC cannot display - it is going into some sort of blanking mode instead. This provides a good separation line for the scrolled characters. After a couple of rasterlines (I used 12) the normal text mode is turned back on. To avoid jumping of the lines, it is necessary to force a badline. Luckily this is done via the same register as for the ECM and bitmap mode, so the following code does the trick (raster1 is the rasterline where the blanking area starts):
lda #$78 + ((raster1+1) & 7) ;turn on ECM and BMM and cause a badline at line raster1+1
As a welcome side effect, this also sets the 9th bit of the next raster line IRQ.
Unfortunately, the ECM+Bitmap blank does not cover the sprites, to cut off a sprite in mid-drawing I temporarily switch the screen pointer to a location where
SCREEN+$3f8 are pointing to an empty sprite. Since there is no output of characters, the other content of the screen area does not matter. I used $0000 as screen address during blanking and had the empty sprite at $340 (so address $3f8 to $3ff contain $0d):
sta $d018 ;change sprite pointers by switching screen
That's it to start the blanking bar, two memory writes. To end the bar, I set these two addresses back after a couple of rasterlines. In my experiment, I used a rasterline IRQ to start blanking at line 150 (this is in the middle of a character) and ended the blanking bar in 162. The gray border showing in the picture is an additional effect that can be left out.
One disadvantage is that the bar created with ECM+BMM is always black, but for my purpose this is acceptable.