"when so requested" is correct and usable in written English.
You can use it when you are indicating a certain condition or response that is dependent upon having been asked to do something. For example, "The employee will provide all requested documents when so requested."
However, we are naturally also in the business of obtaining motion-picture writing assignments for our clients when they so request.
[T]here's an intriguing wrinkle in the race for Mr. Emanuel's seat: talk that he really only wants to serve as chief of staff for two years, and wants a mere seat-warmer who would give the job back when Rahm so requests.
Do so when requested to begin the restoration process.
So when Lowe Enterprises requested permission to build 50 condominiums, which the company calls casitas, in an oceanfront resort called Terranea in this lush section of southwest Los Angeles County, the commission demanded restrictions.
So when he requested that I play some tunes I began to anxiety-sweat at the thought of choosing an artist that he didn't cool-approve of.
So when I requested my £20 refund, I was told that it wasn't there because it was, of course, only virtual money and I could therefore claim back nothing at all.
Many Democrats have said the law should be left as-is, and although President Barack Obama has voiced support for changes, he didn't propose doing so when he requested $3.7 billion to deal with the influx of more than 57,000 unaccompanied minors who have crossed the border illegally since October.
So when requesting help, health centres are less likely to mention the problem".
He was given a remit to conduct inquiries into possible breaches of the ministerial code, but could do so only when requested by the prime minister, who retained the power to decide whether a minister should keep his job if a breach was committed.
