[latepaul]

The following "wrapper" batch file will convert the 8.3 filename back to the long version and then call ebook-viewer.exe with it:

@echo off
REM VIEWER.BAT - a wrapper for Calibre's ebook-viewer.exe 
REM 
REM This script expects a filename in 8.3 format, it will 
REM convert it to a long filename and call ebook-viewer

if "%1" == "" (
	echo VIEWER.BAT - Missing filename
	goto :eof
)

REM get the directory name of the filename passed in
FOR /f  "tokens=*" %%A IN ('echo %~dp1') DO (
   set DIR_NAME=%%A
)

REM convert filename to long format
FOR /f  "tokens=*" %%A IN ('dir /b %1') DO (
   set LONG_NAME=%%A
)

REM pass the full filename to ebook-viewer
REM Change the path to Calibre if necessary
"C:\Program Files (x86)\Calibre2\ebook-viewer.exe" "%DIR_NAME%\%LONG_NAME%"

I've knocked this up quickly and done minimal testing. Use at your own risk.