DeCollibus' name does leap to mind for left handers, but his technique is highly unusual. Off the top of my head I can't think of any videos of someone writing plain Spencerian at all, the more's the pity.
Schin has done quite a few on business writing with fountain pens which are very worthwhile, but as for arm movement and posture, sorry - as you suggest the norm does tend to be an overhead view of a disembodied hand at work.
I have heard nothing but good about Harvest Crittenden's courses. Several people I know have taken them, and all have found them valuable. That would appear to be far and away the best option for distance learning. If you do get a chance to meet up with with a very good penperson in real life, however, jump at it, even if it's just for half an hour.