I agree that with a narrow gap and a large hose there will be a local increase in pressure on the walls (forms), but to me it seems it would be a function primarily of flow rate, viscosity (slump) of the concrete, and depth of concrete above the end of the hose. The pressure would affect the flow rate, but if I understand correctly, with a pumper, that would be a function of the pressure at the pump, the difference in height from the pump to the end of the hose, and the losses due to friction in the hose. For calculation purposes, it should be independent of the height it rises to, since it's a closed system. Granted, if the high point of the system is higher, greater pressure may be required at the pump.
All that to say, in order to limit the pressure on the formwork and prevent blowouts, the flow rate and the depth of the hose into the concrete must be limited.
Presuming that there will not be any water in the forms, there really isn't any reason to have the hose in the concrete. It should not be more than a few feet above the filled depth to prevent segregation of the concrete mix, and this too would increase pressure on the formwork due to impact.