It's a frustrating situation but think about it from the lab manager's point
of view.  Essentially you are asking for custom cropping at mini-lab prices.
Are you planning on standing in the middle of the production area, looking
over the shoulder of the operator while they bring each image up on their
monitor and then have them wait for your input on the cropping of each
individual image?  Taking down a machine, probably their only printer,
capable of printing (for the sake of argument) 40 rolls of film per hour at
a net profit per roll to the lab of (for the sake of argument) $2.00 per
roll (that's only $80.00 per hour and I suspect most labs net more than
that!).  To print a couple of lousy rolls shot with crappy (from the point
of view of the lab!) cropping and make the lab only $4.00 net per hour
instead of $80 per hour.

My response (were I still running a mini-lab) would be, yes sir! Mr.
Customer, glad to oblige, AND BRING LOTS OF MONEY!

Steven, you said, "Enlarger cropping fifty 4"x6" prints is not an option."

Is that because enlarger cropping is slow and expensive?  It's also slow and
expensive to manually input cropping for individual prints on a printer (one
of the reasons that reprints cost more than run of production prints with
original orders).

And be ready to explain to all the other customers shooting SLR's with
decent viewfinders why their orders are late.  I had a guy with $10,000
worth of Leicaflex gear try that with our lab once, we finally had to ask
him to leave, he wanted to color correct each print as it came out of the
machine and have us do it over 3-6 times for 25 cents per shot--note that's
25 cents for the ones he'd take not 25 cents for each print we made on his
quest to the perfect print.  He started shooting color slide film (at my
suggestion) and was surprised to learn that the color that's there (in
Mother Nature's paintbox) is rarely as the shooter remembers it at the time
he made the images.

The most you should expect from a mini-lab is in-focus prints with decent
color balance and exposure, consistent from print to print (within a series
shot with the same light), neatly trimmed, professionally packaged and
delivered when promised.  Anything more is a bonus.  Mini-lab managers are
usually good photographers (some of the best I've known were NOT), mini-lab
owners rarely are good shooters and the operators are people that will work
for what the owner will pay (in my area 7.00 per hour--that's less than
McDonald's).

Note that:

1-high speed flash synch is rarely needed when shooting in darker confines
such as parties
2-many inexpensive RF's with between the lens shutters have viewfinders with
parallax correction
3-shooting flash with slower shutter speeds will get you more ambient fill
light and atmosphere

(My Nikon 8008s SLR's will synch at 1/250 but I rarely shoot faster than
1/30 when shooting low light events, I find "dragging the shutter" gives me
more ambient light for fill and more "ambiance", my flash duration for close
up is about 1/10,000th of a second no matter what shutter speed I select on
camera, I use the TTL flash mode with my flash, composition is very accurate
and the electronic rangefinder aid is pretty accurate.  Note that I do not
use autofocus but I do use the electronic focus aid at times, I will also
preset focus many times before swinging the camera up to my eye and just
shoot--faster is better for candids.)

If you can't adjust your technique or change your equipment you'll probably
have to change you expectations of what you can get from a mini-lab.  Len's
suggestion is a good one:  Better that you crop your scanned negatives at
home and burn a new CD, take it to Walmart (etc.) and get digital reprints.

--
darkroommike

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