Hopefully I can now help others in these situations. Having read the above posts it's rather convincing but conflicting advice. I can now understand why because there's a lack of consistency with the Indonesian banks so people would've had different experiences.
So I'm British currently in the UK and just had to pay $150 US and 200,000 Rp for my VITAS. I (well my wife as sponsor) got the email saying the dollar payment can only be done at BNI, BRI or Mandiri. Firstly I must say this is ridiculous, they know I'm a foreigner in a foreign country, how do they expect people like us to pay in Indonesia? Would it be that difficult to accept credit cards?! Luckily like others here my other half has family in Indonesia who were happy to help.
So one of my sister in laws went to BRI in Tarutung where she has an account and they told her she could not send the $150 dollars by converting it from Rupiah in her account and she could also not give them the dollars in cash from a money changer to send that way. The only option was to open a USD denominated account and send from there. So open an account just for this one transaction! We passed on that option. We had the converstaion on Whatsapp call speaker so there was no miscommunication.
So my other sister in law in Duri went to BNI where she has an account and they said that because they are a small provincial branch of the bank they don't have "the machine" to send dollars. She couldn't send it from her Rupiah account by convertion. It was a long confusing conversation and sounded like giving the dollars in cash would be ok, but I wouldn't bet on that. Turned out my sister in law needed her bank book to proceed anyway and she only had her atm card on her, so the plan was to go home and get it and get some Rupiah changed to dollars on route back to the bank.
On the way when she was returning to BNI with bank book she was passing a Mandiri bank, she doesn't have an account there but called in anyway. They said no problem they can do it, she doesn't need US dollars cash they can convert from Rupiah cash. The only snag was she needed her KTP identity card and her tax number which she didn't have on her. So she went home got the tax number, returned to bank and money sent no dramas. So where as Kiperto stated above that Mandiri wanted the US dollars in cash because no bank account was held, that was not our experience. His experience was 4 years ago though, so maybe Mandiris policy has changed.
So it seems that there is variation in the options available at BNI, BRI and Mandiri, perhaps even between individual branches of the same bank. Also it is probably best to go to as big a branch as possible, like in a main city as the small regional branches might not have "the machine" and maybe staff training to send dollars. Hope this helps some people out in the future, just send someone on a mission to all 3 banks until you get success basically. Good luck!