Spalted timbers can be far weaker than non-spalted timbers but there is no way to gauge whether or not they will fail under load (the veins are actually rotted timber and therefore weak points). I have seen spalted beech explode a few times on the lathe when being turned and in the UK the most commonly seen spalted logs seem to be beech. The spores contained within the timbers are also potentially harmful and can cause respiritory distress when sanded, sawn or planed. For those reasons I'd avoid like the plague in this application
Instead there are some superb home-grown feature timbers which may be suitable such as olive ash, brown ash, tiger oak (stripey), pippy oak, quilted sycamore, birds eye maple (not seen much in the UK - mainly imported but some hose grown id seen from time to time), lacewood plane, walnut, yew, etc to name a few