Firstly: 

The fuqaha’ are agreed that it is haraam to have intercourse with a menstruating woman in her vagina, because Allaah says (interpretation of the meaning): 

“keep away from women during menses and go not unto them till they are purified (from menses and have taken a bath).”

[al-Baqarah 2:222]

And the Prophet (peace and blessings of Allaah be upon him) said: “Do everything except intercourse.” Narrated by Muslim (455). Al-Nawawi narrated that there was consensus on this point. 

Secondly: 

It is haraam to have intercourse with a woman in her back passage, because Allaah says (interpretation of the meaning): 

“And when they have purified themselves, then go in unto them as Allaah has ordained for you (go in unto them in any manner as long as it is in their vagina)”

[al-Baqarah 2:222]

i.e., in the front passage, which is the place of tilth. This is confirmed by the verse in which Allaah says (interpretation of the meaning): 

“Your wives are a tilth for you, so go to your tilth, when or how you will”

[al-Baqarah 2:223]

The back passage is not the place of tilth. 

There are ahaadeeth which indicate that it is haraam to have intercourse with a woman in her back passage. It was narrated that Abu Hurayrah (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Cursed be the one who has intercourse with his wife in her back passage.” Narrated by Ahmad and Abu Dawood (2162); classed as hasan by al-Albaani in Saheeh Abi Dawood. 

See also question no. 1103. 

Thirdly: 

The scholars have stated that it is permissible to enjoy that which is between a woman’s thighs and between the buttocks, without penetrating the back passage. 

Imam al-Shaafa’i said in al-Umm (8/257): 

With regard to enjoying what is between the buttocks without penetrating, there is nothing wrong with that. End quote. 

Ibn Qudaamah said in al-Mughni (7/226): 

There is nothing wrong with enjoying that which is between the buttocks without penetrating, because the Sunnah forbids the back passage, and applies only to that. And it is haraam because it is harmful, and that applies only to the back passage, so the prohibition applies only to that. End quote. 

It says in Rawd al-Taalib:

What the husband is allowed to enjoy of his wife: 

He may enjoy everything except her anal sphincter, even that which is between the buttocks. As for enjoying the anal sphincter it is haraam to have intercourse there, because of the report which says: “Allaah is not too shy to tell the truth; do not have intercourse with women in their back passages.” Narrated and classed as saheeh by al-Shaafa’i. End quote. 

It says in Asna al-Mataalib (3/185): 

The words “As for having intercourse in her anal sphincter…” – such as if part of the tip of the penis disappears into it. End quote. 

Based on this, there is nothing wrong with what was asked about, subject to the condition that there be no risk of the tip of the penis or part of it entering the back passage, for that is haraam. 

And Allaah knows best.