Word to describe an engraved slogan or name for charity purposes?

I believe you are looking for dedication.

The third definition given by Merriam-Webster is

Dedication   noun

the act of officially saying that something (such as a new building) was created for a particular purpose (such as worship) or to remember or honor a particular person

Perhaps "commemorative donations" will work. It seems to be used by other charities soliciting money and offering some way to memorialize content from the donor. I believe you will want to use some derivation of the word "commemoration."

http://dictionary.reference.com/browse/commemorate

Perhaps "commemoration for charity" would work.

Here are sites soliciting money using the term "commemorative donations" (the first stresses that "It is regretted that money donated cannot be used for the provision of an individual personalised tree or seat," but the others stress the personalization of the sort you mention):

http://bradgatepark.org/memorial/commemorative-donations/

https://www.vbgardens.org/support/commemorative-donations/

https://www.vbgardens.org/support/commemorative-donations/

http://www.hoytarboretum.org/support-hoyt/commemorative-donations/

For example, from Hoyt Arboretum:

"Silver Leaf Stewards

The Silver Leaf Stewards program is the perfect way to honor your family or pay tribute to someone who has touched your life in a significant way. With your tax-deductible gift, a metal leaf plaque will be installed on a rock wall in the arboretum engraved with your honoree’s name."