You can extend the VDC to 27 lines, then manually clear and write the bytes to those two lines of text. Make a little routine to read the byte values of the function keys and place them in the VDC ram locations for those two lines. If you don't change anything else in the controller, those two lines will remain static. Correct me if I'm wrong as it's been a few years since I fiddled with that.
Ok - understand I that it needs to extend lines to 27 and then use WINDOW command from Basic performing it in assembler? And for actual values + WINDOW check create IRQ?
No need for anything else actually: no irq no window no nothing like that. The BASIC editor will not go past 25 rows. You can use that fact by extending the VDC to 27 lines and directly writing to those addresses. The editor will never use those rows normally--Nothing else is needed. Now, I'm not sure if you need to move attribute ram down a bit to provide for those extra two lines or not...