[RTTY] FT-1000 Mark V Sub Receiver IF Filter

[W3OA]

I'm trying to decide if I should buy an IF filter for the Mark V sub 
receiver. I'm considering the INRAD 500 khz crystal filter.

I wonder if  it's possible to select the sub receiver narrow filter when 
in RTTY and/or Packet modes?

The operating manual seems to say this filter can only be selected in CW 
mode but that seems unusually restrictive to me. Maybe there's a work 
around?

Thanks for any help - Dick, W3OA