I'm simply dubious about whether the RHS have fully tested this over a period of years OR thought through any other effects it might have.
An advantage of your proposal is that you could trim or fully hack back the ivy occasionally. perhaps half each year.
OR I have this and it is very vigourous, flowers for ages the remains attractive. we have pigeons (Posh name doves) and sometimes robins in ours on a trellis.
It could almost be known as the 'mile-a-minute' clematis, such is the rapid growth of this variety. Clematis montana 'Mayleen' is the best scented Clematis montana too, with delightful satin-pink ruffled blooms. As if that wasn't enough, it has sparkling bronze foliage too. If you want a quick-growing handsome climber, look no further! Height: 10m (33'). Spread: 3m (10'). Pruning Group: 1.
For the mesh, Id get a plastic coated one, as it looks decent when bare.