I've emailed the text of the panel which appears in every single issue
of our Photoworld magazine directly to flyfisher. For others, here it is:

DYNAX xi and si cameras, and most later models, have a shutter lock,
which prevents the shutter from being released if anything other than an
AF lens is fitted (e.g. slide copier, T2 manual mount lens, telescope
etc). To over-ride this –
Dynax 9/7    Select Custom Function 16 and set to 2
Dynax 60    Select Custom Function 13 and set to 2
Dynax 5    Select Custom Function 14 and set to 2
Dynax 4     Select Custom Function 12 and set to 2
On other Dynax models, press and hold the following buttons, while
turning the main switch to ON:
Dynax 40        DRIVE
Dynax 3L        SUBJECT PROG and DRIVE
Dynax 9xi/7xi    FUNC and AEL
Dynax 5xi        FUNC and SPOT
Dynax 800si    AEL and SUBJECT PROGRAM
Dynax 700si    CARD and SPOT
Dynax 600si    LENS RELEASE and ISO
        (with lens removed for safety)
Dynax 500si    AV and DRIVE/ST
Dynax 500si Super    SPOT and DRIVE/ST
Dynax 300si    FLASH MODE and DRIVE/ST
Dynax 505si/Super    SPOT and SELF TIMER
Dynax 404si    P and SELF TIMER
        (with Function Dial at ME position)
Dynax 303si    MODE and SELF TIMER

Dynax 2xi       
Dynax 3xi        Send to Minolta (????!!!) for circuit
Dynax Spxi    modification (chargeable)

Dynax 7D/5D and Alpha 100 use menu command ‘Shutter Lock’

RD-175    SELF TIMER and AV
RD-3000    DRIVE and SPOT (while switching to REC)

On the Vectis models, press the following buttons. There is no need to
‘press and hold’
Vectis S-1        MODE + SELF TIMER = ‘on’ appears on display; SEL = ‘off’
appears on display. Press any other button to confirm.
Vectis S-100    SUBJECT PROGRAM and +/-   = ‘on’ on display. +/- = ‘off’ on
display. Press any other button to confirm.
To re-instate the shutter lock, repeat the same procedure. On Dynax 9,
7, 5, and 4 set Custom Function 16 back to option 1.

Someone else has answered your question, but I'm going to piggyback
your post with a related question:

I've got a similar setup (in my case a KM 5D and 500mm Celeston MF
fixed-aperture f8.0 lens). But is there any way to get the camera to
automatically select the right exposure or tell me what sort of
exposure to use? I've only had the camera a few days, but I found that
if I just stick the lens on and set the camera mode to Auto, the
exposures were all over the place.

The only way I could get decent shots was a lot of trial and error:
Eventually I put it in Shutter priority mode and adjusted the shutter
speed (and ISO) until things looked about right. Is there some way I
could use the meter to tell me? I'm guessing the problem with Auto is
that the camera doesn't know what the aperture of the lens is?

Trial and error is obviously easy with digital, but it would be nice to
get a little help from the camera.

- PG