Sliderule,
I see, and specifiy, HP sections used as walers for sheet pile shoring all the time. Though they are not designed for this, they generally are the most econmical as pile driving contractors often have stacks of different HP sections sitting in thier yard.
DWHA
Stitch welding walers to the sheetpiles is usually not possible. Even when driving frames are used, sheet piles do not go in straight. You are likely to have gaps of an inch or more where welds are requied. If the piles are driven wild, chances go from slim to none.
However, walers are generally supported at intervals by I or T stubs welded to the pile. Welding the walers to the stubs can provide adequate bracing.