Actually this scenario can occur far more then you seem to believe. You use the stormtrooper for capping or decapping the enemy cutoff, fuel or victory point, and then plop it there a minute or two for the inevitable recap. This is not an uncommon tactic with ambush units.
You claim that pioneers and stormtroopers overlap, but somehow royal engineers and commandos don't. You mentioned commandos can get brens, well so can royal engineers, albeit weaker variants just like pios have weaker mp40s then stormtroopers. As already mentioned, conscripts and combat engineers is far more egregious in this regard, yet nobody is complaining about how those two have literally identical weapons.
If your goal is just to force the enemy to reposition, incendiary grenades do just as good a job as standard ones, except that incendiary grenades can deny buildings while regular grenades wont.
Then ambush camouflage would do the same thing for pgrens, probably better with their better range.
As I said, Pioneers need a damage buff, thus lack of identity. Royal engineers have a better target size and have a non-doctrinal size increase.
Bundle grenades would, again Pgrens do the same job but better.