Dog barking at visitors

[GinAndOnIt]

I think I'm asking for the holy grail really, but I'll give it a go.

GinDog dog is 4 years old, and very much a farm dog. He's always lived in a secluded farmhouse with an outdoor run and kennel if needed, and lots of outside space. He has never been deterred from barking at unusual noises and strangers when at home because it was vital for security. (Frequent trespassers, arsonists and burglars, and the barking is a great deterrent)

Anyway, we've moved house into another farmhouse, but this time with no outdoor kennel or run. Nice big garden and fields behind, but only the back garden is totally blocked in, so he's not allowed to be out by himself out the front. The back garden doesn't allow for him to see who's coming to the door, whereas the previous farmhouse did.

In the previous house, he would bark until he saw the person (and our reaction to the person) and then stop. He would have stopped by the time the person was coming to the gate, so it's never been a problem. Now, though, if someone knocks the door, he barks and tries to get out of the front door when I open it, or will bark in whatever room he's in if I close the door to keep him away. He stops as soon as he knows we're okay with the person at the door, but he (obviously) sounds intimidating.

I don't want to stop him barking completely, as security is still an issue, and he still spends time at previous farmhouse where the barking is incredibly useful (especially if I am there alone). But on rare occasions when we have nervous visitors, or children coming, it would be so handy to have a signal for him not to bark and frighten them.

I have tried putting him in the back garden when a visitor is expected, but he is worse like this because he can't see who it is. He will then bark until I show him, or jump up at the back door to try to see. The same will happen if I lock him into a room until the visitors are settled. At the moment, I just tell anyone coming that he will bark but won't jump up/cause harm/whatever, but as someone who used to be terrified of dogs, I do sympathise with how unnerving it can be when you are greeted by barking. If anyone is coming who especially dislikes or is scared of dogs, I will send him off to the farm while they're here, but most of the children love him after 5 minutes, but don't like that first noisy introduction.

Is there anything I could do, do you think? As I say, I really don't want to stop the barking completely, but would like to be able to teach him to stop if asked.