I've often heard it said that in Ibn Taymiyyah's opinion, apostasy (without involving seditious acts) is a crime for which there is no earthly punishment. I haven't seen a reference for this though. In which work did Ibn Taymiyyah mention this (if at all)?
This is not true. It was a false claim made my Dr. Hisham Kamali and he has been proven wrong from Ibn Taymiya's own literature.
Ibn Taymiya made his views very clear in his book "As-Sarim al-Maslool 'ala Shatim ar-Rasool" 3/696
على خصوصها وكلاهما قد قام الدليل على وجوب قتل صاحبها والادلة الدالة على سقوط القتل بالتوبة لا تعم القسمين بل انما تدل على القسم الاول كما يظهر لمن ذلك تامل الادلة على قبول توبة المرتد فيبقى القسم الثاني وقد قام الدليل على وجوب قتل صاحبه ولم يات نص ولا اجماع بسقوط القتل عنه والقياس متعذر مع وجود الفرق الجلي فانقطع الالحاق والذي يحقق هذه الطريقة انه لم يات في كتاب ولا سنة ولا اجماع ان كل من ارتد باي قول او اي فعل كان فانه يسقط عنه القتل اذا تاب بعد القدرة عليه بل الكتاب والسنة والاجماع قد فرق بين انواع المرتدين كما سنذكره
شیخ الإسلام ابن تيمية رحمه الله - الصارم المسلول على شاتم الرسول، الجزء ٣، صفحة ٦٩٦
"...Apostasy is of two types: ordinary apostasy and extreme apostasy, for which execution is prescribed. In both cases there is evidence that it is obligatory to execute the apostate, but the evidence indicating that the sentence of death may be waived if the person repents does not apply to both types of apostasy. Instead, the evidence indicates that that is allowed only in the first case (ordinary apostasy) as will be clear to anyone who studies the evidence that speaks about accepting the repentance of the apostate. In the second type (extreme apostasy) the obligation to put the apostate to death still stands, and there is no text nor consensus to indicate that the death sentence may be waived. The two cases are quite different and there is no comparison between them. It does not say in the Quran or Sunna, or according to scholarly consensus, that everyone who apostatizes in word or deed may be spared the death sentence if he repents after he is a captured and tried. Rather the Quran and Sunna, and consensus, differentiate between the different kinds of apostates."
As you can see, he clearly believed in the death punishment.