We have a site where in the footer we have been asked to change our links from the sister companies name to a keyword of a product that they sell, they theorize that this will rocket the term up the charts for them. So for example they would like to change Widget Inc to Spark Plugs and then the link would go to Widget Inc home page. What we don't want is the anchor text to be indexed for us, but we want to pass link juice to the sister sites. Is nofollow or noindex a better solution or is there no way of really doing it?
If you have anchor text in a link on a page, then the anchor text (whether text or alt text) is seen to be part of that page.
AFAIK there is no way to say 'Don't notice this text as part of our page but only as part of the page we're linking to'.
And, when you think about it, as far as relevancy for your page is concerned, why would it be worse for your page to have text that says 'product' rather than 'company' in the footer? Neither are on topic.
Finally, changing sitewide footer links might be dicey. Not saying it won't work, but you have to conside the possibility that you'll trigger a filter. In your instance the link stays the same but the anchor changes, which is perhaps less of a signal than a completely new link, but depending on size of the site it's potentially a big signal.