Hmmm.....
There are alternative services in West & South Yorkshire [eg an EC from Leeds 5 minutes behind at 0530 for Wakefield and Doncaster; a 0600XC from Leeds and Wakefield for Sheffield and services from Doncaster to Sheffield at 0540 & 0600]. Now passengers from Wakfield and Doncaster for London will presumably use the faster EC trains, so are you saying the 50 or so boarding at Wakefield & Doncaster are for Derby, Loughborough, Leicester or Wellingborough?
Regarding timings - I think you are a bit generous. It should be possible to leave Leeds about 0500, Huddersfield 0520, travel non-stop to Barnsley about 0600, and arrive in Sheffield about 0622 to depart as booked at 0627. [or even include a pick-up only at Brockholes, say, to accommodate the Holme Valley traffic colpepper and VideozVideoz envisage!].
If the 50 or so on the train from Wakefield and Doncaster are actually going south of Sheffield, [but not to London], then I'm sorry, they will be inconvenienced. But I still can't help feeling that a through service from Huddersfield, England's 10th largest town and with a population of 146,000 [ source Wikipedia; and just bigger than Wakefield & Doncaster put together] -would be worth a go, even if it does mean earlier than ideal departures from Huddersfield and Barnsley. And a through service back at 1815 off St Pancras is surely worth a try, too.
It would mean EM crews learning Leeds-Barnsley via Huddersfield, but presumably they already learn other routes off the core services [eg. Doncaster-York and Scarborough] for a once-a-week operation, so would this add too much to their workload?
Finally, yes - it would mean them going earlier in the morning from Sheffield to Neville Hill, but then I always assumed trains were run for the convenenience of passengers, not crew!

