I suspect that different publishers have different ways of describing the instruction you have in mind, which (if I understand it correctly) consists of two parts:
Don't set the attribution/source credit on a separate line unless there isn't room to include all of it on the final line of the block quote.
Align the attribution/source credit on the right-hand margin of the line where it appears.
The publisher I worked for many years ago in New York would have formulated the instruction something like this:
Run in the attribution, and set it flush right.
The "run in" part of the instruction indicates that the typesetter shouldn't put the attribution on its own line following the block quote. The "flush right" part of the instruction indicates that the typesetter should not follow regular character spacing ("justification") in placing the attribution after the last words of the block quote, but should instead set the attribution flush to the right margin.
Whether the typesetter is human or mechanical, you should also be able to specify that the attribution should not be broken across a line; so, for example, if the quotation is attributed to "Henry Wadsworth Longfellow," and a natural line break happens to occur immediately after "Wadsworth," the typesetter will know to move the entire name to the final line, flush right.