The four times I've been seconded over the last 20 years it has always been to "act up" and come with a temporary pay bump. Usually because I'm taking over a manager's job while they're on maternity leave / until a new one can be found because they've quit, or because a team could do with one of my weird skills for a few months. In my experience people either stay at the same pay or their pay goes up while on secondment. Secondment is generally supposed to be temporary so "overpaying" someone for a few months is generally worth it, especially when you consider the costs of onboarding a new employee.