'Abdu'l-Bahá regularly attended prayers at mosques and presented himself and his followers as Muslims.

'Abdu'l-Bahá regularly attended prayers at mosques and presented himself and his followers as Muslims. You might want to read "The Young Turks and the Bahá'ís in Palestine" by Necati Alkan, an instructor at the Wilmette Institute. Here is a quote from page 264: "Throughout their stay in Ottoman domains – until the death of 'Abdu'l-Baha in 1921 – the Baha’is presented themselves to outsiders in the Middle East as followers of Islam...'Abdu'l-Baha consequently states that the Americans were guided to Islam (ihtida) through the teachings of Baha’u’llah. Initially, American Protestant missionaries in Iran engaged in religious conversations with the Baha’is and joined the Baha’i tarikat. The new converts returned to America, propagated the Baha’i teachings and won over many Americans in a short time. Later, some Americans who travelled to Palestine met ‛Abdu’l-Baha, and this resulted in their belief in the ‛manifest religion of Muhammad’ and they recognized ‛the unity of God, approved and confirmed the prophethood (nübüvvet) of His Holiness, the glory of the Messengers [Muhammad], and believed in the greatness (ulviyet) of my late father Baha’u’llah’

Regarding dissimulation, in October 1921, 'Abdu'l-Baha encouraged the Bahá’ís of Manchester to emulate the freemasons...

October 1921
In the Name of God! O Sincere Servant of the Blessed Beauty!
Your detailed letter has been received, and has been the cause of the utmost joy and gladness. Praise be to God! The loved ones are all astir and active, but prudence is necessary. You have rent the veil too widely asunder. Explain to the loved ones that the rending of the veil to such an extent will be the cause of great agitation, and the harm thereof will reach to the Holy Land. Great caution is necessary. Discourses in churches and great public gatherings are in no wise permitted as in this place enemies, within and without, are lying in wait and are bent on aggression. Prudence requires that activity should, for the present, be concealed and carried on with the utmost moderation. Convey to the loved ones, one and all, on my behalf, the greatest longing, love and kindness. Give a spiritual message from me to Mr. Healds and say unto him: “Peruse the Gospel, how His Holiness Christ – may my life be a sacrifice to Him – says, ‘Conceal it, that the Pharisees may not be informed thereof.’ Now the same condition prevails.”
This matter is of the greatest importance. On no account let them contribute articles to the newspapers, and so long as they are not sure of any soul, let them breath no word to him. Consider how the Freemasons have for two hundred years carried on their work, and unto this day they have not openly declared it to any soul. Not until they find a hearing ear will they speak. The loved ones too must proceed with the greatest prudence, lest serious difficulties be created. If any one should travel to the Holy Land, he must on no account declare to anyone by the way that his purpose is to visit us. The loved ones must, in the presence of strangers, speak forth simply the teachings of the Blessed Beauty and mention no word of the belief concerning Him. Should anyone inquire, “What is your belief regarding the Blessed Beauty?” let them answer: “We regard Him as the foremost teacher and educator of these later ages and Abdu’l-Baha as the Centre of His Covenant.”

The original scanned document can be found at http://www.h-net.org/~bahai/docs/vol13/Barstow_600-623.pdf , where it is labeled BC#608.
It was published through H-Net's Documents on the Shaykhi, Babi and Baha'i Movements.