as a basis for a plugin I would use this workflow...
copy selection into new drawing...
move all axes to centre,centre, bottom [e.g. TT_axes tools]...
create a proxy with same axes, a single line with the length of the most used element and hidden square at one or both ends [to allow scaling]...
select all and replace with proxy...
scale each proxy to intersection points...
this workflow could possibly be automated, but would still require tweaking by eye and addition of missing bits...
tree steps...
trusses-sample-centerlines_001.skp
john