I think videomapping plugin has some great ideas listed on the wishlist.

Basically you get a long way if you can see the video, moving marker and perhaps some adjustable guide lines to hint the angle of view as a reference. Constant speed, fast/slow speed and picture by picture would be needed (as well as the audio track).

Sometimes it might be great to have two cameras such as forward and rear facing cameras, so multiple windows would be great. Calibration should be finer than 1s which is coarse at faster speeds.

The script I'm using (on my linux box) is like this:

videfile=$1

echo Extracting frames at 1Hz rate
ffmpeg -i $videofile -y -ss 1 -an -sameq -f image2 -r 1 pic%05d.jpg

# Get video date/time from exif info
echo Initially set time/date for all files at the beginning of the video file
exiftool -CreateDate="`exiftool -CreateDate -S $videofile | sed 's/.*: //' `" *

num=0 #num of seconds to add # Files are at 1s difference
for file in pic*.jpg
do
exiftool -CreateDate+=0:00:$num $file # adjust file 1 +1s, file2 +2s, ..
num=$(( $num + 1 ))
done

Kew Garden (London) has a very high metal walkway along (or even slightly above) the tree tops. osm.org/?lat=51.475743&lon=-0.297338&zoom=18&layers=M

Is is tagged as footway + bridge + layer. Height would be nice tag too.

For a long route Garmin seems to route by preferring the large roads (high road class in mkgmap). Near the start and finnish it has to use smaller roads as well as when the large roads are not connected. I guess Garmin uses this to make the road network smaller to make the route calculation time smaller. Putting it in other words, it seems to ignore smaller roads.

Unfortunately this means a lot of options like "avoid highways" or "shortest" are meaningless when you calculate the route for a network consisting of motorways and major roads only.

If you mark some via points for the route you can guide to smaller roads because it has to use them to reach the via point. This makes the segments shorter too ie. it uses more small roads in calculation. (More smaller roads are used for a short segment)

I've tried this with a tweaked map. I made all cycleways road class 4 in mkgmap (ie. like motorways). That map routed to cycleways (in cycling mode) perfectly. Because of the large amount of "major roads" the routing calculation time on my Oregon was useless though.