There are two main questions here:
- Can you direct your estate to apply a copyright license to your work?
- Is the resulting license irrevocable?
In the case of (1), the answer is very likely "yes," but you should consult an estate attorney about forming a trust or trust-like entity to hold and (possibly) administer the copyright. Simply directing the estate to apply a license to the copyright may not be effective, because a copyright license is not considered a transfer of property, and an estate is not an appropriate vehicle for applying such licenses.
In the case of (2), this is a complicated mess because of 17 USC 203. In short, you must hold the copyright personally until you die, and then bequeath it to the trust (or other entity) in your will. Some estate attorneys will try to transfer the copyright into the trust before you die. That doesn't work correctly, because the transfer is not pursuant to a will and does not qualify as a bequest. As a result, it is not eligible for the exemption in the aforementioned law, and so it gives rise to so-called "termination rights." Termination rights will eventually entitle your surviving children and/or spouse to revoke the copyright transfer to the trust and any licenses the trust has subsequently issued. Termination rights cannot be assigned or disposed of in your will - they are always inherited by the surviving spouse or children (or by various other people such as your executor, if you have no surviving spouse or children). They also cannot be waived by any provision of the license (the GPL doesn't even try, nor do any other well-known licenses, because the statute explicitly says that such waivers have no effect). There is little case law as to how termination rights affect licenses offered to the general public, such as the GPL, but there is no provision in law barring their application, except that the termination must identify the specific grantee whose rights are terminated. This likely makes it impractical to terminate all instances of the license, but even one well-placed termination of a large company's GPL rights might be enough to destroy an open source project, if that company provides most of the developers for that project.
Note that termination rights apply to all copyright licenses and transfers, except for bequests by will and works for hire. Any time you place any copyrighted work under any license, or transfer it by any means other than a bequest or a work-for-hire arrangement, it always gives rise to these termination rights, so there's nothing special about the "apply a license after I die" fact pattern, other than the bequest exception. For further discussion of this issue, see my answer on Open Source.SE.