Paint Tool SAI has no shift-constraints when using a linework vector layer to draw with. This my preferred workaround. Gives me great flexibility to wing my perspective.
This may already be a well-known thing among SAI users, but I thought I'd make it into a mini-tutorial anyway
This may be the start of a new mini-series of tutorial which focuses on a single aspect of using a piece of software. PROTIPS!
Edit: Added a new point about adding a quick curve. (#4)