Made whole thing a wee bit better (fullscreen in epubs) and pushed it out to people here:
https://www.mobileread.com/forums/showthread.php?t=90085
In worst case, I'll have some new bug reports 8)
There's one thing that I got stuck at: flipping an option consists of closing all files opened by uds, changing the db, then reopening last file. I reopen that file by sending startTask to sysd, asking it to start 'uds /media/mmcblk0p1/thatlastfile'. Problem is, if I do it this way, I lose 'Back to Books' option in the popupmenu. I saw that reader itself seems to be toggling that option on and off with setItemLabel message to popupmenu - but even if I explicitly enable it, it returns me to main reader screen upon activation. Does anyone know whether I can set this menu entry to behave properly? Or is it some bit of logic located in uds that I have to convince? :)