What form you use is irrelevant. If you area certified or licensed appraiser you can write the value on the back of a napkin and you have made an appraisal. You can even give it to them verbally and you have made an appraisal. However, if you make an appraisal without following the needed steps to have a credible opinion of value you are in violation of USPAP. The easier way to handle this would be to tell the client you want to use a form that is designed to report value. However, if you want to please them, you can do the same appraisal work (and charge the same price) that you would do with another form and report the value on the form they requested. Just be sure you keep adequate documentation of the work you have done in your work file. You will, of course have to write a narrative comment to report the value on this form. Because of that, I would consider charging them extra to use this form– that will probably solve your problem.
You could also use the same process I do when I get a request for a 704. I tell the client my fee for a 2055 will be $zzz but if they insist on a 704 it will be $100 more. Haven't had to use the 704 since 1996! Your reply could be a fee quote for a 2055 exterior only or if they want a value with the 2070 then the fee would be $100 more than the 2055 exterior! Bet it the order gets upgraded to a 2055 exterior at leaset.
you need to remember that it is our (your) responsibility to provide a report that is not misleading. Just because the client asks for you to complete the report on a 2070 form doesn't mean you do. As professionals it is our job to provide the client with what they need even if it differs from what they request. Simply inform your client that by definition, the 2070 is a marketability report and does not address the issue of value.