As it's been pointed out, in DOO areas you have full cab secure radio so you don't get dead spots like that. More to the point, in many areas where guards operate they do so because there is Absolute block signalling, with no track circuits over many miles. Therefore track circuit clips are absolutely pointless (as they are in axle counter areas, say most of the WCML) and if there is no communication the procedure is to run down the track with detonators and flags, like something straight out of the Railway Children and based around practices from around that time.
I repeat I am for additional station on trains (where possible) to perform customer service and revenue protection duties. But the whole practice of guards is something straight from the 19th century and has been long since technically superseded by something better.
Ideally, you would have 15 control centres, communicating with every train via GSM radio. On train staff would know there whereabouts via GPS or triangulation in problem areas such as tunnels. All staff should be carrying around the rule book, full timetable, their position on the network, sectional appendixes and notebooks on PDAs.
On train staff, if they don't already, should be concentrating on duties like selling tickets, answer queries, clearing rubbish, helping with bags and wheelchair passengers, even selling food and drinks. They should not be in the back cab operating doors, nor should they be running off down the track with flags.
There is no reason why they cannot be recruited straight from a retail background, sent on a conversion course for a few weeks, with a grounding in the duties they need, including evacuation procedures. If something happens to the driver, there is nothing stopping them hitting a distress alarm on the GSM radio, assuming a DOO DSD alarm is not generated anyway. I wouldn't sack or reduce any current staffs rate of pay, but new recruits could be recruited for less, and have at least 1/3 of their pay made up of performance bonuses based around good customer service, revenue protection, keeping trains clear of rubbish, and selling tickets and other items.
It may take a long time to happen, but if it saves money and maintains safety it should happen.