> Can anyone tell me if (and how) it is possible to start Remote Link from
> within OPL?

For the S3a, you can use code like the following:

PROC islink:
  local ax%,bx%,cx%,dx%,si%,di%,fl%
  local link$(14),pid%,cline$(8)
  link$="ROM::LINK.IMG"
  cline$="-b19200"+chr$(0)    rem must get leading byte count right
  bx%=addr(link$)+1
  cx%=addr(cline$)
  di%=addr(pid%)
  ax%=$0100        rem for FilExecute
  fl%=os($87,addr(ax%))
  if fl% and 1
    print "Error:",err$(ax% and $ff)
  else
    bx%=pid%
    ax%=$0600      rem for ProcResume
    fl%=os($88,addr(ax%))
    if (fl% and 1)
      print "Error:",err$(ax% and $ff)
    else
      print "Link started and has PID",hex$(bx%)
    endif
  endif
  get
ENDP

For the S3, there's no LINK.IMG in the mother rom so you have to specify 
LOC::C:\IMG\LINK.IMG as the filename to run instead.  And you'll have to 
specify a slower baud rate (the -b parameter).

If you try to use this code when Link is already running, you'll get a 
misleading result, since Link will happily start again, and then 
terminate itself (with "File already exists" error).  So you'll in 
general also need a function like

PROC islink:
  local ax%,bx%,cx%,dx%,si%,di%,fl%
  local link$(8)
  link$="LINK.*"
  bx%=addr(link$)+1
  ax%=$0100      rem for ProcIdByName
  fl%=os($88,addr(ax%))
  if fl% and 1
    print "Link not running"
  else
    print "Link running and has PID",hex$(ax%)
  endif
  get
ENDP

Finally, in order to stop link, use ProcTerminate (O/S service $0d88), 
passing BX as the PID of the process to terminate, and 0 in AL.

Regards, DavidW