Good questions and answers all,
I'm with BOAC on this one!
In my limited recent experience - Boeing, AB and other passenger carrying aluminium tubes all have 'standby' AIs in the shape ISIS/ISI or a Stby AI plus an RMI (or sommat similar) and they are usually have independent power supplies for gyros and air data feeds etc.
However, the question is - when was the last time we practiced using said standby instruments? Its a bl**dy tricky scan - even to maintain S and L at height. Furthermore, SA can be difficult to acheive without RMIs/BDHI and DME/ILSs etc. Loss of any power/thrust indications only makes matters worse.
Sadly, I don't think there there is any requirement to practice/test any sort of skill on these rudimentary insts - I'm not sure it was even part of the Boeing/Airbus initial training (and it did not crop up in some previous companies' recurrent trg)- surely this is wrong? However, in a previous life and although there was no legal/staff/company requirement to do so, I used to get myself, and our pilots/ FIs to practice a descent, arrival and an SRA/ILS on standby insts once a year - it always produced some interesting debrief points!
Next time you fly, imagine such an approach on a dark and dirty night with limited battery time left!