Christain,

You could do what they call pin propping, which as you say is using acro props. They knock small holes thro the fire place above the position of the new lintel and put timber or steels through the holes. Then use the acros to support the timber/steel that pases thro the hole.

In the real world I am sure you will be fine and if it was my fireplace I would strip the plaster back acces the condition of the mortar/brickwork to make sure its not completely had it. Knock a hole through where you want the new lintal (gently) install said lintal then remove the old lintal below and remaining brickwork.

Just apply a bit of common sense mate and you will be fine.